The degradation kinetics of Direct Blue 15 (DB15), a diazo dye, were studied over a suspended and immobilized TiO2 catalyst. For all experiments, the kinetics experiments were performed in a swirl flow photoreactor under the influence of UV light. The effect of different parameters: dye concentration, catalyst loading, and light intensity, on the DB15 kinetics was investigated. The kinetic rates were assessed using apparent ka approach, a single value of reaction rate kr and adsorption constant K, and approach of kr as of variable. The DB15 mineralization was discussed as well. Using a dip-coating device, the P25 catalyst was deposited on a Pyrex glass. The thin film surface characterization was examined. The coated catalyst was evaluated by checking the effect of two variables: initial dye concentration and light intensity on the DB15 kinetics. In terms of the ka approach, the results demonstrated that DB15 degradation is described by the pseudo first-order kinetics model. The Langmuir-Hinshelwood (L-H) model was fitted well with the experimental data for the number of process variables. L-H constant kr was determined as a function of three parameters: initial dye concentration, catalyst loading, and light intensity. The ka values were evaluated and compared with experimental results. In terms of three variables, ka can be expressed as ka=0.15 [C]o−0.69 [W]0.73 I0.91 1 + 0.17 Co  while the empirical model results in the following expression, ka=0.77 Co−1.65 W0.73 Io0.89. It was observed that 83.64% mineralization was achieved after a period of 16 h. In terms of immobilized catalyst, the DB15 degradation kinetics was described by a pseudo first-order model for different dye concentrations. Meanwhile, a power-law model described the impact of light intensity on dye kinetics. In addition, the coated catalyst was successfully reusable with high efficiency for up to four cycles.

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Photocatalytic degradation of Direct Blue 15 (DB15), an azo dye, was studied using a swirl-flow monolithic reactor under UV irradiation. The degradation reactions were carried out to investigate effects of initial dye concentration, catalyst loading, and light intensity at an optimal pH. The experiments were designed and mathematically modelled by CCD-RSM (central composite design-response surface methodology) approach. It was found that the selected parameters significantly affect DB15 degradation. In terms of the linear term, catalyst loading and light intensity had a synergistic effect, while dye concentration registered the opposite effect. Strong interaction was observed between catalyst loading and both light intensity and initial dye concentration compared with the interaction of light intensity and initial dye concentration. Based on the experimental results, a quadratic model was developed to predict the percentage removal of DB15. The predicted values of the model were in good agreement with the experimental values (R2 = 0.987), indicating the model fits well for the parameter space for which experiments were performed. According to diagnostic plots, the model credibility was valid because its residuals were distributed normally and exhibited a random pattern based on their examination versus the predicted values. The results revealed that the initial dye concentration and catalyst concentration have a significant effect on the mineralization time.

In different things, including foods and various materials, gaining certain colours is generally obtained by adding substances widely known as dyes and pigments [1,2]. In the applied medium, water or oil, dyes are soluble materials primarily used for textile materials, whereas pigments are typically insoluble substances utilized via a dispersion method to obtain different products: inks and paints [1][2][3][4]. Different dyes are utilized for various product colorations in the textile industry, with an estimated amount between 7×10 5 and 7×10 7 tons worldwide annually [5,6]. The dye structure generally consists of two main constituents, namely, chromophore and auxochromes. The function of chromophores is light absorption to produce a certain colour. Hence, they are fundamentally responsible for the dye colour. Examples of chromophore groups are azo (-N=N-), carbonyl (C=O) and methine (-CH=). Meanwhile, auxochromes have two different functions: playing a role in colour enhancement, more precisely, increase the colour intensity and a role in dye attachment to the surface of the applied medium, in the textile industry, attached dye to the fiber surface. In dyes, the auxochromes are classified into two main groups: acidic and basic groups. While acidic auxochromes can include -OH, -COOH, and -SO3H, the basic groups of auxochromes involve -NH 2 , NHR, and -NR 2 [1-3,7-8]. 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,

The release of massive pollutants amounts continuously because of urbanization and industrialization has caused a big ecological problem worldwide. Due to their activities, effluents of many industries: mining operations, paper/pulp, and batteries, release different heavy metals, including Copper (Cu), Lead (Pb), and Nickel (Ni), into the environment. Heavy metals are of big concern due to their high toxicity, big bioaccumulation susceptibility, and serious threat to humans and ecosystems. Compared to organic pollutants, which are highly influenced by biological and chemical degradation, heavy metals have no degradation into end products using these methods. Therefore, the removal of such metals is considered a big challenge in water purification. For metals removal, different techniques have been applied, such as precipitation, ultrafiltration, and coagulation. However, those applications have many drawbacks: low-efficiency, high consumption of reagents, and generation of toxic sludge. In contrast, adsorption considers an effective method for metal removal, owing to the method’s simplicity, economic and versatility, making it the most convenient way for toxic metals removal. Many conventional adsorbents, such as activated carbon and alumina, have been effectively employed. Nevertheless, the biggest disadvantages of using such adsorbents appear to be their price due to high activation costs and limited reusability. Biosorption has lately emerged as a method with several advantages, including minimal cost, ease of use, and great efficacy, even for trace amounts of metallic ions. This paper aims to review the relevant literature regarding the adsorption method for heavy metals removal from wastewater. Different treatments of heavy metals from wastewater and their related features are highlighted. The metals’ toxicity and hazards to health and the environment are also discussed. The application of various materials as bioadsorbents is explored, such as natural adsorbents and industrial and agricultural wastes.