Fe(III)–oxalate complexes induced photooxidation of diethylstilbestrol in water

In this work, the photooxidation of diethylstilbestrol (DES), a synthetic estrogen, was investigated in a concentric reactor under a 125 W high-pressure mercury lamp (λgreater-or-equal, slanted365 nm). The photooxidation efficiencies were dependent on the pH values and Fe(III)/oxalate ratios of the system, with higher efficiency at pH 3.50 ± 0.05 and Fe(III)/oxalate ratio 10.0/120.0 μmol l−1. The photooxidation reactions obeyed the law of pseudo-first-order reaction at the concentration over the range of 2.0–10.0 mg l−1 of DES. The photooxidation rates decreased with increasing the initial concentrations of DES. For 2.0 mg l−1 DES, the observed photooxidation rate coefficient (kobs) was 0.00622 min−1. By using GC–MS and LC–MS techniques, the predominant photooxidation products DES-o-catechol ([M]+, m/z 284) and DES-4-semiquinone ([M]−, m/z 267) were respectively identified and the mechanisms for the oxidative degradation were proposed. When DES reacted with OH radicals, C atoms in 3-position were added with OH radicals to produce hydrolyzed DES radical followed by two oxidation pathways: (1) dehydrolyzing to produce DES-4-semiquinone which was oxidized further to DES-4,4′-quinone; (2) undergoing peroxidation by O2 and getting rid of HO2 radical to produce DES-o-catechol. After that, the two H atoms on the hydroxy group of the catechol were extracted in two individual steps to form intermediates semiquinone radical and o-quinone. The catechol intermediates underwent further oxidation, benzene ring cleavage and decarboxylation, up to mineralization ultimately.