Citation: Safri, A.; Fletcher, A.J. Concentration Dependence of TiO 2 Nanoparticles in Carbon Xerogels on Adsorption–Photodegradation Applications. Gels 2023, 9, 468. https://doi.org/10.3390/ gels9060468 Academic Editor: Zhi Li Received: 24 April 2023 Revised: 23 May 2023 Accepted: 1 June 2023 Published: 7 June 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/). gels Article Concentration Dependence of TiO 2 Nanoparticles in Carbon Xerogels on Adsorption–Photodegradation Applications Anam Safri and Ashleigh Jane Fletcher * Department of Chemical and Process Engineering, University of Strathclyde, Glasgow G1 1XJ, UK; anam.safri@strath.ac.uk * Correspondence: ashleigh.fletcher@strath.ac.uk; Tel.: +44-141-5482-431 Abstract: A suite of composite materials comprising carbon xerogel content and TiO 2 was synthesised via a modified sol–gel method. The textural, morphological, and optical properties of the composites were extensively characterised and correlated with the observed adsorption and photodegradation performances. The homogeneity and porous structure of the composites depended on the amount of TiO 2 deposited in the carbon xerogel. During polymerisation, Ti-O-C linkages were formed, which favoured the adsorption and photocatalytic degradation of the target methylene blue dye. Adsorption was deemed favourable, and most accurately fitted by the Sips model, exhibiting a maximum uptake of 209 mg g −1 estimated for the sample containing 50% TiO 2 . However, the synergistic effect of adsorption and photocatalytic degradation for each composite depended on the amount of TiO 2 deposited in the carbon xerogel. The dye degradation process for the composites with 50%, 70%, and 90% TiO 2 improved by 37%, 11%, and 2%, respectively, after exposure to visible light after adsorption. Repeated runs demonstrated over 80% of activity was retained after four cycles. Thus, this paper provides insight into the optimal amount of TiO 2 required within such composites for maximum removal efficiency via adsorption and visible light photocatalysis. Keywords: carbon xerogels; photocatalyst titania; adsorption; photodegradation; adsorption isotherm; recyclable composites; water treatment 1. Introduction Increasing water pollution, along with the appearance of emerging pollutants, has led to ongoing developments in innovative wastewater treatment methods that can meet the standards for clean water. Environmental catalysis is one such technology that can effectively respond to this demand, and these methods can be enhanced by developing new materials and processes to meet the needs of an increasingly industrialised society. Among these technologies, photocatalytic processes are interesting systems, which are occasionally used in combination with other techniques to improve water treatment processes. In particular, since these systems use visible irradiation, they can save energy, allowing them to be employed in developing countries. In this regard, the photocatalyst TiO 2 has been reported in combination with other materials to enhance its photocatalytic performance. One way to improve photoactivity is by combining TiO 2 with an adsorbent material, whereby the synergistic effect of the integrated materials enhances pollutant adsorption and disintegrates pollutants under visible light [1]. Carbon-based adsorbents are often used as adsorbent materials because of their high surface area and porous nature, which facilitate the adsorption of pollutants [2]. Addition- ally, carbon materials have been employed to modify the electronic structure of TiO 2 to improve visible-light photocatalysis because TiO 2 has a large bandgap and is only acti- vated upon UV irradiation to generate electron and hole pairs, which undergo a series of chemical reactions to produce hydroxyl species responsible for disintegrating pollutants [3]. Additionally, carbon materials can entrap photoexcited electron and hole pairs, inhibiting Gels 2023, 9, 468. https://doi.org/10.3390/gels9060468 https://www.mdpi.com/journal/gels