Recent Advances in the Photocatalytic Degradation of Direct Blue Dyes: A Critical Minireview and Analysis Study Recent in the Photocatalytic Degradation of Direct A Critical

Dyes are categorized into various classes depending on different aspects. However, the classification based on their usage and chemical structure are the most popular ways. Depending on the type of applied method to the used substrate, dyes are categorized into different groups. Such groups include Acid Dyes, Basic Dyes, Direct Dyes, Reactive Dyes, Disperse Dyes, Solvent Dyes, Sulfur Dyes, Vat Dyes and Mordant Dyes [3,8]. Among different dye types, Direct Dyes have great importance due to both their characteristics and applications. In terms of their applications, Direct Dyes are applied with no requirements for the fixing process, precisely without the step of affixing agent. This leads to a simplified dyeing process and lowered process operating costs. Their applications include paper, ABSTRACT Different dyes are used in numerous industries, including food, paper, ink, and the textile industry. Among various dye types, Direct Dyes have great significance because of their properties and applications. Such applications are paper, cotton, leather, and cellulose. The textile industry is considered a main contributor to the water pollution problem because it releases large quantities of coloured liquid waste. The majority of dyes and their products are considered carcinogenic, toxic, and resistant compounds. Therefore, the application of common wastewater treatments is not sufficient for dye removal such as adsorption, biological and chemical degradation. The advanced oxidation process using photocatalytic oxidation is an effective technique that can mineralize complicated aromatics and different organic dyes among various treatment methods. In this review, the photocatalytic degradation of Direct Blue dyes is discussed. The review is mainly focused on the recent advances in the photocatalysts and their activity improvement using different dopants. Minireview and Analysis

cotton, leather, cellulose, and nylon. These dyes are typically watersoluble with a strong affinity to cellulose fibers. Their solubility is enhanced due to the presence of sulfonate groups in some types.
Though such substances are anionic dyes, their categorization is not under the acid dyes class. This is because the attachment to the fiber surface does not usually occur by acid groups [8][9][10][11]. The textile industry waste is a major contributor to aquatic life pollution among different waste types due to releasing a huge amount of coloured wastewater to the environment. The reason behind such significant wastewater amounts is the features of the dyeing process itself. The dye bath tank's draining and cleaning after each dyeing process are the two main factors responsible for releasing coloured wastewater [12]. As the dye's usage is dramatically increased due to their numerous applications, the effluent of their usage, especially textile industries, is considered a big challenge in the wastewater treatment area. This is because of various factors: high levels of Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) [13,14]; most dyes and their by-products are carcinogenic and highly toxic compounds [15][16][17][18][19] and difficult to be completely degraded because they are more resistant substances for the typical water treatment methods such as adsorption, photolysis, biological and chemical degradation [20][21][22].
Among different alternative treatments, applying the Advanced Oxidation Process (AOP) using the photocatalytic oxidation approach is a promising method for resistant materials such as complicated aromatics and organic dyes. The use of the photodegradation technique leads to degrading highly toxic compounds to environmentally friendly minerals [3,20,[23][24][25][26]. Therefore, this way has been attracted many researchers. This minireview paper addresses the application of AOP using the photocatalytic degradation approach for Direct Blue dyes. This paper's main focus is discussing the recent advances in photocatalytic degradation in terms of using different photocatalysts and their modifications using various dopants.

Literature Review
In terms of Direct Blue dyes, El-Bahy, et al. [27] examined the photocatalytic degradation of Direct Blue dye (DB53) using various lanthanide ions doped with TiO 2 . Under UV irradiation, the study demonstrated that the type of dopant has a high degree of impact on the catalyst surface properties such as texture structure, particle size and bandgap. Gd-TiO 2 appeared to be the most effective catalyst, resulting in the highest dye removal due to its excellent surface properties. Using a Cu 2 O catalyst, it was observed that the hydrothermal temperature has a key role in determining the main properties of the synthesized catalyst, including the bandgap, shape, and surface area. In terms of DB53 degradation, the catalyst with a nanorod shape was highly effective compared to other catalyst shapes [28]. In contrast, Mohamed, et al. [29] changed the hydrothermal time for preparing YVO 4 nanoparticles from 4 h to 24 h. Their study found that the YVO 4 nanoparticles with a size of 11 nm exhibit the optimum behaviour of catalyst activity. Sobana, et al. [30] applied 2% Ag-doped TiO 2 for the degradation of DB53 using both UV and solar light irradiation.
The application of solar irradiation for the Ag-doped TiO 2 registered a higher DB53 degradation than UV light utilization.
Sobana, et al. [31] concluded that the catalyst activated carbon AC-ZnO has a good activity for DB53 removal under solar light irradiation. While alkaline pH appeared to be more preferable than acidic pH, catalyst grinding showed a negative impact on DB53 degradation efficiency. Mohamed, et al. [32] observed that the In terms of reaction rate, the obtained results showed that DB53 dye degradation is described by pseudo-first order kinetics using different catalyst types [29][30][31][32]. DB71 was degraded under UVC irradiation. The findings suggested that high dye removal is achieved for two different cases: UV irradiation and UV/ TiO 2 . In terms of pH solution and temperature, there was little effect on DB71 degradation Saien, et al. [34,35] [34].
In contrast, the obtained data of DB71 degradation was accurately fitted by pseudo-first order reaction kinetics Boumaza, et al. [38] As a part of their study, Habibi, et al. [41] investigated the degradation of DB160 solution using commercial TiO2 catalyst.
With a UV source of 400 W, the obtained results found that complete decolorization is successfully achieved. With respect to the reaction kinetics, pseudo-first order kinetics clearly described the DB160 degradation. It was also observed that inorganic ions have an inhibitory effect on the DB160 degradation rate Saroj, et al.  By applying the sol-gel method, ZnO nanoparticles were prepared using Arabic gum (AG) as a template. Under visible light, it was observed that using ZnO nanoparticles leads to a high dye removal up to 95% Fardood, et al [43].
With respect to DB15 dye, Lamba, et al. [44] synthesized CeO 2 -ZnO nanodisks using a precipitation method. In a double-walled reactor, an aqueous solution of 50 mg/L DB15 was irradiated using natural solar light for 4 h during the time between 10 am to 2 pm. Their study found that the CeO 2 -ZnO catalyst shows excellent activity that could possibly be due to an enhanced recombination rate Ebrahimi, et al. [45]  However, the higher dopant concentrations have no effect on the catalyst photoactivity.

3.
The dopant type influences the photocatalyst surface characteristics such as surface texture, particle size and energy bandgap.

4.
With different photocatalysts, the photodegradation of Direct Blue dyes is well described by pseudo-first order kinetics.