The Chemical Background of Advanced Oxidation Processes

In advanced oxidation processes, the hydroxyl radical is the main initiator of the degradation of pollutants. For aromatic molecules, the rate coefficients are between 2×109 and 1×1010 mol−1 dm3 s−1 and show some variation according to the electron-withdrawing or donating nature of the substituents. The one-electron oxidant .OH induces 2–4 electron oxidations of many aromatic pollutants. These high rates are explained by .OH addition to an unsaturated bond, scavenging of the organic radical by dissolved O2, and subsequent reactions. In amino-substituted molecules and in azo dyes the efficiency is lower, because O2 cannot compete efficiently with the unimolecular transformation of carbon-centered radicals. Generally, the toxicity first increases and then decreases with treatment time. The increase is attributed to the high toxicity of some degradation products and to H2O2 formation. In surface waters, traces of transition metal ions degrade some of the H2O2 in Fenton-like processes.