Advanced oxidation processes (AOPs) are a promising solution for wastewater treatment, known for their efficiency in degrading organic pollutants. Among various AOPs, peracetic acid (PAA)-based technologies have attracted significant attention as an alternative to hydrogen peroxide (H2O2). Unlike H2O2, PAA has a longer peroxide bond and lower bond dissociation energy, making it more easily activated to produce abundant radicals, including hydroxyl (•OH), acetylperoxy (CH3C(=O)OO•), acetoxy (CH3C(=O)O•), and methyl (•CH3) radicals. In addition, secondary radicals like hydroperoxy (•OOH) are also formed during reactions in water.
UV light is commonly used to activate PAA in AOPs, with different wavelengths of UV light triggering different radical generation pathways. However, the direct detection and identification of these radicals, especially alkoxy and alkyl radicals, is a great challenge in this area. Traditional quenching experiments cannot provide conclusive evidence on the presence of these radicals, while electron paramagnetic resonance (EPR) technology can determine radical signals. However, accurate identification of these signals still remains difficult due to the variety of radicals involved. Recent advancements in computational methods, particularly density functional theory (DFT), have offered powerful tools to deeply investigate the underlying mechanisms of radical formation and transformation. Specifically, DFT calculation can provide new insights into the electronic structure and charge distribution, allowing a better understanding of excited-state transitions contributed to radical generation.
In this study, researchers developed an in-situ EPR set-up to accurately identify the radicals generated by PAA activation under different UV wavelengths (185, 254, and 365 nm). The results showed significant differences in the types and concentrations of radicals depending on the UV wavelength. UV light at 254 nm led to the formation of several key radicals, including •OH, CH3C(=O)OO•, CH3C(=O)O•, and •CH3. When PAA was exposed to UV light of 185 nm, higher concentrations of •OH were produced, along with additional CH3•C(=O). In comparison, UV light of 365 nm only generated a small amount of •OH. By combining EPR spectroscopy with DFT calculations, the researchers clarified the different activation mechanisms under the three UV wavelengths. The study highlighted that the difference in radical generation was due to the local excitation mechanism at 254 nm and the Rydberg excitation mechanism at 185 nm. These findings provide deep insights into the radical generation process in UV/PAA systems and are valuable to optimizing PAA-based AOPs for water treatment.
This research not only contributes to the understanding of radical formation in PAA-based AOPs but also offers a more accurate method for identifying the radicals. The combination of experimental and theoretical approaches provides a comprehensive view of the mechanisms involved in UV/PAA activation, helping to improve the efficiency of UV-based AOPs and expand their applications in environmental remediation.