## 3 Results and Discussion
### Solid-Phase Extraction of Dyes  
In the course of this work, studies were carried out that made it possible to evaluate the ability of copper terephthalate to adsorb dyes in aqueous solutions. A study was made of the effect of the initial concentration, pH, contact time on the adsorption of dyes, and the kinetics of the process. With an increase in the initial concentration, the degree of adsorption increased significantly, and with an increase in temperature, it decreased (Figure 6). The initial pH of the dye solution is a key factor controlling the adsorption process. The pH was studied in the range from 3.0 to 11.0, as shown in Figure 8. The degree of the extraction of dyes increases with increasing pH from 3.0 to 7.0. However, when the pH was between 7.0 and 11.0, the recovery decreased. It can be concluded that the surface of  
Figure 6: Dependence of solid-phase extraction on the initial concentration of the dye (**a**—SY, **b**—TAR, **c**—Orange II, **d**—Methyl Orange). Figure 7: Comparison of dye solutions and dye adsorbed on the sorbent (**a**—SY, **b**—TAR, **c**—Orange II, **d**—Methyl Orange). copper terephthalate can acquire a positive charge, which can interact with anionic dyes via various binding mechanisms and pathways, such as electrostatic attraction or hydrogen bonding. Dyes are best extracted in an environment close to neutral. However, in an acidic environment, extraction processes are better than in an alkaline one. The results obtained in the study of the adsorption of various azo dyes on copper terephthalate were analyzed using various adsorption models, particularly the Langmuir and Freundlich models. The scheme of the Langmuir isotherm is based on the hypothesis of monolayer adsorption, where all sorption centers are dynamically and identically equivalent [33]. The linearized Langmuir isotherm can be expressed using Equation (3):  
\[\frac{C_{e}}{q_{e}}=\frac{1}{q_{max}K_{L}}+\frac{C_{e}}{q_{max}} \tag{3}\]  
where \(K_{L}\) is the Langmuir adsorption constant (mg/L), and \(q_{max}\) is the maximum adsorption capacity of the monolayer (mg/g). The Freundlich isotherm shows adsorption on a heterogeneous surface with the same energy and in the presence of interactions between adsorbed molecules. This means that it is not limited to building a monolayer. The linearized Freundlich isotherm can be expressed using Equation (4):  
\[\log q_{e}=\frac{1}{n}\log C_{e}+\log K_{F} \tag{4}\]  
The parameters of the Langmuir and Freundlich adsorption isotherms are summarized in Table 1.  
Figure 8: Dependence of the degree of extraction on the pH of the medium (**a**—SY, **b**—TAR, **c**—Orange II, **d**—Methyl Orange). High \(K_{F}\) and 1/n values were obtained. This indicates that the adsorption of dyes by copper terephthalate promotes positive cooperative binding and its inherent heterogeneity [34]. The correlation coefficient (R\({}^{2}\)) shows that the adsorption of the Sunset and Tartrazine dyes is better described by the Freundlich isotherm than by the Langmuir model, which indicates that adsorption occurs on a heterogeneous surface with the same energy, interactions between adsorbed molecules are possible, and adsorption is not limited to building a monolayer. However, it can be concluded that the adsorption of the Orange II dye is better described by the Langmuir model, which indicates that all sorption sites are energetically equivalent and identical and that adsorption is based on monolayer adsorption. It was also concluded that both models are applicable for Methyl Orange. To determine the spontaneity of the adsorption process and verify the experimental data, three main thermodynamic parameters were used: the change in enthalpy (\(\Delta\)H\({}^{0}\)), entropy (\(\Delta\)S\({}^{0}\)), and Gibbs free energy (\(\Delta\)G\({}^{0}\)).