materials Article Synthesis of Selected Mixed Oxide Materials with Tailored Photocatalytic Activity in the Degradation of Tetracycline Katarzyna Siwi ´ nska-Ciesielczyk 1, * , Angelika Andrzejczak 1 , Dominik Paukszta 1 , Adam Piasecki 2 , Dariusz Moszy ´ nski 3 , Agnieszka Zgola-Grze´ skowiak 4 and Teofil Jesionowski 1   Citation: Siwi ´ nska-Ciesielczyk, K.; Andrzejczak, A.; Paukszta, D.; Piasecki, A.; Moszy ´ nski, D.; Zgola-Grze´ skowiak, A.; Jesionowski, T. Synthesis of Selected Mixed Oxide Materials with Tailored Photocatalytic Activity in the Degradation of Tetracycline. Materials 2021, 14, 5361. https://doi.org/ 10.3390/ma14185361 Academic Editor: Stefano Lettieri Received: 8 July 2021 Accepted: 11 September 2021 Published: 17 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, Berdychowo 4, PL-60965 Poznan, Poland; andrzejczak194@wp.pl (A.A.); dominik.paukszta@put.poznan.pl (D.P.); teofil.jesionowski@put.poznan.pl (T.J.) 2 Institute of Materials Science and Engineering, Faculty of Mechanical Engineering and Management, Poznan University of Technology, Jana Pawla II 24, PL-60965 Poznan, Poland; adam.piasecki@put.poznan.pl 3 Institute of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów 42, PL-71065 Szczecin, Poland; dmoszynski@zut.edu.pl 4 Instituteof Chemistry and Electrochemistry, Faculty of Chemical Technology, Poznan University of Technology, Berdychowo 4, PL-60965 Poznan, Poland; agnieszka.zgola-grzeskowiak@put.poznan.pl * Correspondence: katarzyna.siwinska-ciesielczyk@put.poznan.pl; Tel.: +48-61-665-3626 Abstract: The elimination of antibiotics occurring in the natural environment has become a great challenge in recent years. Among other techniques, the photocatalytic degradation of this type of pollutant seems to be a promising approach. Thus, the search for new photoactive materials is currently of great importance. The present study concerns the sol–gel synthesis of mono, binary and ternary TiO 2 -based materials, which are used as active photocatalysts. The main goal was to evaluate how the addition of selected components—zirconium dioxide (ZrO 2 ) and/or zinc oxide (ZnO)—during the synthesis of TiO 2 -based materials and the temperature of thermal treatment affect the materials’ physicochemical and photocatalytic properties. The fabricated mixed oxide materials underwent detailed physicochemical analysis, utilizing scanning-electron microscopy (SEM), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), energy-dispersive X-ray spectroscopy (EDS), low-temperature N 2 sorption (BET model), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The synthesized mixed oxide materials were used as photocatalysts in the heterogeneous photodegradation of tetracycline (TC). The physicochemical properties of the fabricated photocatalysts, including morphology, crystalline and textural structure, as well as the pH of the reaction system in the photocatalytic tests, were taken into account in determining their photo-oxidation activity. LC–MS/MS analysis was used to identify the possible degradation products of the selected antibiotic. Keywords: TiO 2 ; ZrO 2 ; ZnO; mixed oxide materials; photodegradation; tetracycline 1. Introduction The dynamic development of civilization and technological progress contribute to the growing problem of increasing quantities of pollutants generated into the natural environment. Among a broad range of such substances, residues of medical and pharma- ceutical preparations and products have been recognized as a significant global problem [1]. Among the wide variety of pharmaceuticals and personal care products (PPCPs), tetracy- clines are the most produced and consumed [2]. Therefore, one of the challenges currently being taken up in many scientific centers around the world is the development of new, effective methods for eliminating or removing various types of pollutants [3]. According to the available literature, methods such as adsorption [4,5], membrane techniques [6], elec- trocoagulation [7], electrochemical methods [8] and advanced oxidation processes [9–11] have been applied for the elimination of certain kinds of antibiotics, include tetracyclines. Materials 2021, 14, 5361. https://doi.org/10.3390/ma14185361 https://www.mdpi.com/journal/materials