Comparative electrochemical degradation of phthalic acid esters using boron-doped diamond and Pt anodes

Phthalic acid esters (PAEs) are a group of endocrine disruptors commonly used as plasticizers. The present study compares the electrochemical oxidation of PAEs at boron-doped diamond (BDD) anode with that at Pt anode. Both the degradation and the mineralization processes of PAEs became much slower when using the Pt anode compared with that using the BDD anode. Moreover, the degradation rates of PAEs decreased at the BDD anode but increased at the Pt anode with increasing alkyl chain length. This was attributed to the different oxidation mechanisms at the two anodes. The BDD electrode has an inert surface that holds a large amount of strong oxidants as free hydroxyl radicals ((*)OH), causing electrophilic attack by (*)OH to be the main reaction. Therefore, degradation of PAEs became slower on the BDD anode due to there being less available electronic energy as the alkyl chain length increased. However, adsorbed oxidants (PtO(x)(+1)) with low oxidation ability tended to form on the surface of the active Pt anode. Therefore, PAEs with longer alkyl chains promote faster degradation owing to their stronger hydrophobicity. Detection of intermediates in the GC/MS tests confirmed the above conclusion, in which oxidation of dimethyl phthalate on BDD occurred on the aromatic ring at first, while the alkyl chain was preferentially attacked on the Pt anode.