Citation: Philippidis, N.; Pavlidou, E.; Sotiropoulos, S.; Kokkinos, P.; Mantzavinos, D.; Poulios, I. Photoelectrocatalytic Oxidation of Sulfamethazine on TiO 2 Electrodes. Catalysts 2023, 13, 1189. https:// doi.org/10.3390/catal13081189 Academic Editor: Vincenzo Baglio Received: 30 June 2023 Revised: 31 July 2023 Accepted: 3 August 2023 Published: 7 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). catalysts Article Photoelectrocatalytic Oxidation of Sulfamethazine on TiO 2 Electrodes Nikolaos Philippidis 1 , Eleni Pavlidou 2 , Sotiris Sotiropoulos 1 , Petros Kokkinos 3,4 , Dionissios Mantzavinos 3 and Ioannis Poulios 1, * 1 Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; nikfilip@hotmail.com (N.P.); eczss@chem.auth.gr (S.S.) 2 Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; elpavlid@auth.gr 3 Department of Chemical Engineering, University of Patras, University Campus, Caratheodory 1, 26504 Patras, Greece; pkokkin@upatras.gr (P.K.); mantzavinos@chemeng.upatras.gr (D.M.) 4 School of Sciences and Engineering, University of Nicosia, 2417 Nicosia, Cyprus * Correspondence: poulios@chem.auth.gr; Tel.: +30-2310997785 Abstract: The photoelectrocatalytic degradation and mineralization of sulfamethazine (SMT), a sulfonamide drug, were explored in aqueous solution. Working electrodes with TiO 2 coatings on Ti substrates (TiO 2 /Ti) were used, which were produced by the dip coating method. TiO 2 film electrodes were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) following annealing at 500 ◦ C for 1.5 h. To photoelectrochemically characterize them, photocurrents vs. applied potential curves were used. The photoelectrocatalytic efficiency (PEC) of the TiO 2 /Ti electrodes regarding the oxidation of SMT has been assessed with reference to degradation and mineralization under different experimental conditions. The selected drug molecule was effectively degraded following the Langmuir–Hinshelwood (L-H) kinetic model. The degradation efficiency was shown to increase with increasing applied potential bias up to +1.5 V vs. Ag/AgCl. It was found to be more favorable in acidic environments compared to alkaline ones. A decrease in the destruction rate constant was recorded when the pH was increased from 3 to 5.6 (natural pH) and 9. The decomposition rate was shown to first increase and subsequently reach a saturation value at high concentrations of SMT, indicating that the degradation also depends on other parameters (e.g., the rate of the charge or the mass transfer on the electrode double layer). The results of the photoelectrocatalytic experiments were compared to those of electrochemical (EC) and photocatalytic (PC) degradation of SMT. A significant enhancement was recorded in the case of the PEC degradation, leading at +1.5 V to an increase of the apparent rate constants of degradation, k, and mineralization, k TOC , of 153 and 298%, respectively, compared to the simple photocatalytic process. Keywords: photoelectrocatalysis; TiO 2 ; sulfamethazine; drugs 1. Introduction Research efforts in the water purification field are continuously growing since water quality control and regulations on different hazardous pollutants have become stricter [1]. Semiconductor photocatalysts offer a simple and cheap process for the abatement of organic compounds under artificial or solar light, and thus an increase in their use has been recorded during the last twenty years [2–4]. Because of its interesting characteristics (i.e., chemical stability against both corrosion and photocorrosion and good photocatalytic activity, particularly in its anatase form), titanium dioxide (TiO 2 ) has been extensively studied and applied, commonly in the form of a slurry of fine particles in a photochemical reactor. The slurry offers a high photoinduced reaction surface and a low recombination rate of the photogenerated e − /h + pairs, resulting in high purification efficiency. However, TiO 2 particles cannot be easily separated from the treated wastewater, requiring high operation costs and a complex treatment system. To prevent the need for a separation step, Catalysts 2023, 13, 1189. https://doi.org/10.3390/catal13081189 https://www.mdpi.com/journal/catalysts