pH-Dependent Activation of EPFRs and Kinetics of Hydroxyl Radical Generation

Environmentally Persistent Free Radicals (EPFRs) pose potential environmental and health risks, yet their behavior remains insufficiently characterized. This study investigates the influence of pH on EPFRs activation during redox cycling, focusing on their capacity to generate hydroxyl radicals (•OH). We hypothesized and confirmed that EPFRs environmental stability is enhanced under low pH conditions, whereas higher pH accelerates their redox activity. A model EPFRs system was employed, consisting of 0.25% copper oxide (CuO) as the transition metal center and 2-monochlorophenol (MCP) as the organic precursor, supported on amorphous silica (Cabosil). Hydroxyl radical production was quantified via DMPO spin trapping across a buffered pH range of 5.5-9.0 at redox cycling intervals of 15, 30, 60, 120, 180, and 300 min. Results revealed that •OH yield per EPFRs (•OH/EPFR) was significantly different on both pH and time (<i>p</i> < 0.001), with substantially higher ratios observed at pH values above 7.4. A kinetic model describing the pH-dependent •OH formation rate was developed, showing excellent agreement with experimental data. The model suggests that hydroxyl radical generation by EPFRs is inversely proportional to proton concentration, exhibiting pseudo-first-order kinetic behavior. These findings suggest that EPFRs persist under acidic environmental conditions but become highly reactive at physiological pH, thereby increasing their potential to produce reactive oxygen species.