Degradation of organic molecules in advanced oxidation processes: Relation between chemical structure and degradability

Hydroxyl radical induced degradation of maleic acid, fumaric acid and 20 aromatic molecules was investigated in air saturated aqueous solutions. Hydroxyl radicals were generated by an advanced oxidation process (AOP), water radiolysis. Oxidation was followed by chemical oxygen demand (COD) and total organic carbon content (TOC) measurements. Up to ∼30–50% decrease of COD the dose dependence was linear. By the ratio of the decrease of COD and the amount of reactive radiolysis intermediates introduced into the solution the oxidation efficiencies were calculated. Efficiencies around 0.5–1 (O2 molecule built in products/OH) found for most of the compounds show that the one-electron-oxidant OH induces 2–4 electron oxidations. The high oxidation rates were explained by OH addition to unsaturated bonds and subsequent reactions of dissolved O2 with organic radicals. In amino substituted molecules or in Acid Red 1 azo dye, O2 cannot compete efficiently with unimolecular transformation of organic radicals and the efficiency is lower (0.2–0.5).

► In oxygenated aqueous solutions OH induces 2–4 electron oxidations.
► Degradation efficiency depends on the nature of the radical formed in OH attack.
► When the organic radical reacts with oxygen with high rate the efficiency is high.
► Formation of phenoxy, anilino, or semi-iminoquinone radical reduces the efficiency.