Performance of the Electro-Oxidation and Electro-Fenton Processes with a BDD Anode for the Treatment of Low Contents of Pharmaceuticals in a Real Water Matrix

Abstract

Here, we report the performance of electro-oxidation and electro-Fenton with a boron-doped diamond (BDD) anode for the treatment of single and multicomponent solutions containing small amounts of pharmaceutical residues (i.e., 1 mg•L-1 paracetamol and/or 1 mg•L-1 salicylic acid) spiked into a real water matrix at pH 3.0. Electro-oxidation was performed in a BDD/Pt cell, whereas electro-Fenton was carried out in a BDD/air-diffusion cell to electrogenerate H2O2 at the cathode, always operating at constant current density. It was found that the decay of both pharmaceuticals by electro-oxidation was more rapid in the real water matrix than in ultrapure water with 0.05 mol•L-1 Na2SO4 because of their additional reaction with active chlorine species produced at the bulk from the oxidation of Cl- ion. Such chlorinated oxidants exhibited even higher reactivity than hydroxyl radicals formed and confined at the anode. The increase in current density largely enhanced the removal of both pollutants. Similar results were found using the real water matrix at natural alkaline pH. When the mixture of both pharmaceuticals was treated by electro-oxidation, their abatement became slower owing to the competitive attack of generated oxidants over them. Only a slight acceleration of pharmaceutical decay was obtained for the real water matrix using electro-Fenton, since the accumulation of additional homogeneous hydroxyl radical formed from Fenton’s reaction between generated H2O2 and added Fe2+ was inhibited by its reaction with Cl- to form much less reactive chlorinated radicals. For the real water matrix with added pharmaceuticals, a high degree of mineralization of the natural organic matter content (NOM) was reached at high current densities by electro-oxidation, which was even improved upon addition of 0.05 mol•L-1 Na2SO4 Traces of oxidation by-products like p-benzoquinone, as well as NO3- and NH4+ ions, were detected during the electro-oxidation of paracetamol solutions, but the N-compounds contained in the real water matrix were not removed under the investigated conditions.