Influence of acid-base equilibria on the rate of the chemical reaction in the advanced oxidation processes: Coumarin derivatives and hydroxyl radical

The decomposition and chemical manipulation of stable aromatic pollutants into less toxic products is an important topic for wastewater management and natural water remediation. The mechanism of the Advanced Oxidation Process (AOPs) of 4,7-dihydroxycoumarin (4,7-DHC) and 7-hydroxycoumarin (7-HC), as examples of stable naturally-occurring industrially-important compounds, in the presence of hydroxyl radical (HO) in the aqueous solution has been analyzed using Electron Paramagnetic Resonance spectroscopy (EPR) and Quantum Mechanics-based test for Overall Free Radical Scavenging Activity (QM-ORSA). The effect of pH values of the medium on the investigated reaction mechanisms has been fully investigated. The rate constants were estimated by the conventional transition state theory (TST) and Eckart's method (ZCT_0). Estimated values of the overall rate constant (koverall) higher than >4.06 × 109 M−1 s−1 at all pH values showed that both compounds undergo a chemical transformation when exposed to HO. When pH increased in the range of 0–14, the koverall increased from 4.06 × 109 to 1.11 × 1010 (4.7-DHC) and 2.09 × 109 to 1.76 × 1010 M−1s−1 (7-HC). At physiological pH = 7.4 value, 7-HC was ∼1.5 times more prone to radical action, as shown by EPR and QM-ORSA, due to the dominant anionic form. Both compounds were more reactive towards HO than Trolox at this pH value. The ecotoxicity assessment of the starting compounds, intermediates and oxidation products indicated that the formed products show lower acute and chronic toxicity effects on aquatic organisms than starting compounds, which is a prerequisite for the development of novel AOPs procedures.