لخّصلي

Growing populations and industrialization have increased water demand, leaving 36% of the global population with inadequate water supplies. Contaminated water is a major problem, especially in low- and middle-income countries. Advanced Oxidation Processes (AOPs), particularly photocatalysis using TiO2, offer a solution by oxidizing organic pollutants. While TiO2's photocatalytic activity was discovered in 1972, its widespread use is hindered by limitations such as a large band gap energy and aggregation. However, modifications like doping and surface modification improve TiO2's light absorption, charge separation, and pollutant adsorption. TiO2-based AOPs are applicable to various industrial wastewaters, including textile, municipal, pharmaceutical, and petroleum waste. Future research should focus on developing cost-effective and reusable TiO2 photocatalysts to enhance sustainability.

Growing populations and economies have resulted in a dramatic increase in demand for water. Therefore, 36% of the global population lives in areas with inadequate water supplies. Contaminated water quality and insufficient water supply are only two of the many water-related difficulties posed by rapid urbanization, especially in low- and middle-income nations. [1], [2], [3], [4]
With increasing industrial activities of civilization, many toxic chemicals are being released in wastewater. Complete removal of non-biodegradable pollutants is challenging to achieve by existing biological treatment processes [2], [5], [6], [7], [8], [9]. Advanced Oxidation Process (AOP) involves the application of free hydroxyl radicals to oxidize the organic contaminants of the effluent [10], [11], [12], [13], [14]. The rapid and non-selective oxidation of organic matter in water is facilitated by photocatalysis, an AOP. In heterogeneous photocatalysis, a semiconductor catalyst speeds up the photoinduced process. Suspending the photocatalyst in solution and irradiating it with light is one of the simplest uses of photocatalysis [15], [16], [17], [18].
Akira Fujishima and Kenichi Honda first reported the photocatalytic activity of nano TiO2 in 1972 [19]. However, its application to environmental remediation started in the 1980 s after Frank and Bard removed cyanide ions from an aqueous solution through TiO2 powder [20]. Since then, researchers are trying to explore effective methods of utilizing TiO2 in effluent treatment [21]. The most effective use of AOPs is the combination of TiO2 and ultraviolet (UV) light. The use of solar irradiation has the potential to improve the process energy efficiency and cost-effectiveness significantly. Researchers have done several investigations to create a visible light-active TiO2 photocatalyst. In order to be activated, the TiO2 photocatalyst must be exposed to UV visible (UV–vis) light. The use of a floating catalyst may achieve proper use of solar irradiation. In order to maximize their exposure to UV light, these catalysts are designed to float on the water's surface [22], [23], [24]. Wang et al. detailed various approaches to synthesizing TiO2, including the sol-gel, solvothermal, and reverse micelle methods. In addition, the processes of TiO2 synthesis have been published, along with the benefits and drawbacks of the techniques used and the environmental applications of photocatalytic products [25], [26].
However, TiO2 has certain limitations that prevent it from being widely used in photocatalysis. TiO2’s non-porous, polar surface, limited absorption capacity for non-polar organic pollutants, and aggregation and agglomeration of catalysts are only a few of the challenges it faces in the field of photocatalysis. Others include the coexistence of electrons and holes in the particle and their greater recombination probability, slower rates of the desired chemical changes in relation to the absorbing light energy, and a relatively large band gap energy (3.2 eV) [27], [28], [29], [30], [31]. Numerous studies on TiO2 modification to improve its photocatalytic capabilities have been reported. These alterations have been made in various ways, such as doping with metals and non-metals, dye sensitization, surface modification, synthesis of composites with other materials, and immobilization and stabilization of support structures. Modified TiO2 always has different qualities from pure TiO2 in terms of how well it absorbs light, how well it separates charges, how well it adsorbs organic pollutants, how well it stabilizes the TiO2 particles, and how easily it can be separated into individual TiO2 particles [14], [32], [33], [34]. Improving the photolytic activity by modifying the structure of TiO2 for the photodegradation of wastewater pollutants has the potential to be an economically viable process, especially for large-scale plants.
Almost every type of industrial wastewater can be treated with the TiO2 process. This includes textile wastewater [13], [30], [35], [36], [37], municipal wastewater [38], [39], coffee-producing wastewater [40], pharmaceutical wastewater [41], [42], [43] petroleum processing wastewater [44], [45], pesticides industry wastewater [46], [47], paper mill wastewater [48], [49], [50], olive mill wastewater [51], [52] and hospital wastewater [53].
The capacity to recycle a photocatalyst is crucial for keeping production costs down, but this problem has not been extensively investigated. The decreased efficiency of photocatalytic activity during reuse may have been caused by the loss of photocatalytic particles to the water or/and the buildup of intermediates with a poisonous impact on the surface of the nanoparticles. Ecofriendly, cost-effective, and reusable approaches to the treatment of polluted water approaches have the potential to be ideal for expanding green photocatalyst systems [23], [54], [55].