Separations 2023, 10, 77. https://doi.org/10.3390/separations10020077 www.mdpi.com/journal/separations Review Recent Clay-Based Photocatalysts for Wastewater Treatment Chitiphon Chuaicham 1 , Jirawat Trakulmututa 1 , Kaiqian Shu 1 , Sulakshana Shenoy 1 , Assadawoot Srikhaow 1 , Li Zhang 1 , Sathya Mohan 2 , Karthikeyan Sekar 1,2 and Keiko Sasaki 1, * 1 Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan; 2 Department of Chemistry, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur 603203, India * Correspondence: keikos@mine.kyushu-u.ac.jp Abstract: Photocatalysis is a remarkable methodology that is popular and applied in different inter- disciplinary research areas such as the degradation of hazardous organic contaminants in wastewater. In recent years, clay-based photocatalyst composites have attracted significant atten- tion in the field of photocatalysis owing to their abundance, excellent light response ability, and stability. This review describes the combination of clay with focusing photocatalysts such as TiO2, g-C3N4, and Bi-based compounds for degrading organic pollutants in wastewater. Clay-based com- posites have more active surface sites, resulting in inhibited photocatalyst particle agglomeration. Moreover, clay enhances the creation of active radicals for organic pollutant degradation by sepa- rating photogenerated electrons and holes. Thus, the functions of clay in clay-based photocatalysts are not only to act as a template to inhibit the agglomeration of the main photocatalysts but also to suppress charge recombination, which may lengthen the electron–hole pair’s lifespan and boost de- grading activity. Moreover, several types of clay-based photocatalysts, such as the clay type and main photocatalyst, were compared to understand the function of clay and the interaction of clay with the main photocatalyst. Thus, this study summarizes the recent clay-based photocatalysts for wastewater remediation and concludes that clay-based photocatalysts have considerable potential for low-cost, solar-powered environmental treatment. Keywords: photocatalysis; TiO2; g-C3N4; Bi-based-compounds; organic degradation 1. Introduction 1.1. Photocatalysis and Its Mechanism A potential global crisis has been brought to light in recent decades due to pollution and energy scarcity. In order to ensure the long-term success of human civilization, it is essential that progress be made in alternatives to polluting and energy-intensive processes as the need to clean up the environment is pressing [1–6]. Since the 1960s, there has been a growing emphasis on photocatalysis as a method for degrading organic contaminants [7,8]. It has been shown to have tremendous promise for CO2 reduction, photocatalytic H2 production, and water pollution purification as a low-cost and environmentally benign technology [9–12]. The photocatalyst is the key component of this technique since it turns solar energy into chemical energy, which, in turn, eliminates the pollutants [13–15]. Figure 1 is the schematic diagram illustrating the mechanism of photocatalysis [16]. Photoexcitation of electrons (e − ) and holes (h + ) happens when the energy of the in- coming light is greater than a critical value. Photon energies are comparable to or greater than the semiconductor’s band gap. Some of the e − and h + at the surface of photocatalysts will interact with electron acceptors or donors. The e − and h + may recombine nonradia- tively or radiatively to generate, but they can also become stuck in shallow or deep traps. When exposed to heat or light, photocatalysts lose part of their photocatalytic efficiency [17–19]. The photocatalyst with poor photocatalytic efficiency in highly diluted solution Citation: Chuaicham, C.; Trakulmututa, J.; Shu, K.; Shenoy, S.; Srikhaow, A.; Zhang, L.; Mohan, S.; Sekar, K.; Sasaki, K. Recent Clay- Based Photocatalysts for Wastewater Treatment. Separations 2023, 10, 77. https://doi.org/10.3390/separa- tions10020077 Academic Editor: Gavino Sanna Received: 27 December 2022 Revised: 19 January 2023 Accepted: 20 January 2023 Published: 22 January 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/license s/by/4.0/).