Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway

Oxidative degradation of aqueous acidic solutions of the beta-blocker propranolol (PPN) has been studied by anodic oxidation (AO) and for the first time by electro-Fenton (EF) process using sub-stoichiometric titanium oxide (Ti4O7) anode elaborated by plasma deposition. The oxidative degradation of the PPN by Ti4O7([rad]OH) formed at the surface of the Ti4O7 anode and [rad]OH generated via electrochemically assisted Fenton's reaction was investigated. Decay of PPN concentration followed pseudo-first order reaction kinetics with degradation rates influenced by both applied current and initial PPN concentration. The absolute rate constant of the reaction between PPN and [rad]OH/Ti4O7([rad]OH) was determined by competition kinetics and found to be (2.99 ± 0.02) × 109 L mol−1 s−1. Relatively high mineralization efficiency of PPN solution (82% TOC removal) was achieved by AO with Ti4O7 anode at 120 mA after 480 min of treatment, whereas almost complete mineralization (96% TOC removal) was reached at similar conditions in EF process. Analogous EF treatment with DSA anode showed lower mineralization (89% TOC removal) compared to Ti4O7 anode. The initial N content of PPN was mainly released as NH4 +, with smaller proportion of NO3 −. Aromatic intermediates such as 1-naphtol; hydroxylated 1-naphtol and phthalic acid were identified by both reversed-phase HPLC and GC-MS analyses. Oxalic, oxamic, maleic and glycoxylic acids were the main short-chain carboxylic acid detected. Based on the identified intermediates, carboxylic acids and inorganic end-products as well as TOC removal results, a plausible reaction sequence for mineralization of PPN by electrogenerated hydroxyl radicals is proposed.