  Citation: Mendes-Felipe, C.; Veloso-Fernández, A.; Vilas-Vilela, J.L.; Ruiz-Rubio, L. Hybrid Organic–Inorganic Membranes for Photocatalytic Water Remediation. Catalysts 2022, 12, 180. https:// doi.org/10.3390/catal12020180 Academic Editor: Gilles Berhault Received: 28 December 2021 Accepted: 26 January 2022 Published: 29 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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 Review Hybrid Organic–Inorganic Membranes for Photocatalytic Water Remediation Cristian Mendes-Felipe 1,2 , Antonio Veloso-Fernández 1 , José Luis Vilas-Vilela 1,2 and Leire Ruiz-Rubio 1,2, * 1 Macromolecular Chemistry Group (LQM), Physical Chemistry Department, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), 48940 Leioa, Spain; cristian.mendes@bcmaterials.net (C.M.-F.); antonio.veloso@ehu.eus (A.V.-F.); joseluis.vilas@ehu.eus (J.L.V.-V.) 2 BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain * Correspondence: leire.ruiz@ehu.eus Abstract: Mismanagement, pollution and excessive use have depleted the world’s water resources, producing a shortage that in some territories is extreme. In this context, the need for potable water prompts the development of new and more efficient wastewater treatment systems to overcome shortages by recovering and reusing contaminated water. Among the water treatment methods, membrane technology is considered one of the most promising. Besides, photocatalytic degradation has become an attractive and efficient technology for water and wastewater treatment. However, the use of unsupported catalysts has as its main impediment their separation from the water once treated. With this, providing the membranes with this photocatalyzed degradation capacity can improve the application of photocatalysts, since in many cases their application improves their recovery and reuse. This review describes the general photocatalytic processes of the main inorganic nanoparticles used as fillers in hybrid polymeric membranes. In addition, the most recent hybrid organic–inorganic membranes are reviewed. Finally, the membranes formed by metal–organic frameworks that can be considered one of the newest and most versatile developments are described. Keywords: hybrid membranes; photocatalysis; MOF; polymer 1. Introduction The never-ending growth of the world’s population together with the major interna- tional problem of water contamination caused by industrial, domestic and environmental influences put global water resources at risk. During the last decades, the freshwater scarcity has become a wide world problem, estimating that around two-thirds of the global population live under severe water scarcity conditions [1–3]. In this context, seawater desalination, non-conventional water resources (rainfall-runoff) and the remediation of wastewaters or marginal-quality water have arisen as necessary alternative resources to fulfill the freshwater needs on a near future of the world population [4]. Wastewater treatment, sometimes called water remediation, is defined as “the applica- tion of known available technologies to treat wastewater to such an extent that the quality of the treated water meets the specifications of governmental environmental regulatory agencies” [5]. These processes are normally classified into five different treatments: pre- liminary, primary, secondary, tertiary and advanced. These transform wastewater to pure one by applying sieves, sedimentation and coagulation methods, biological treatments or disinfection, among others. Particularly, processes such as adsorption, ion exchange, membrane and advanced oxidation of wastewaters grab considerable attention of many researchers worldwide due to the increased number of contaminants detected in water [5,6]. Although the contaminants found in water are very varied, they could be easily di- vided based on their chemical composition into inorganic and organic pollutants. Inorganic pollutants, such as oxyanions/cations and metal ions, especially heavy and/or radioactive Catalysts 2022, 12, 180. https://doi.org/10.3390/catal12020180 https://www.mdpi.com/journal/catalysts