# Solid-Phase Extraction of Organic Dyes on Mixed-Ligand Zr(IV) Metal-Organic Framework
## 3 Results and Discussion
### Solid-Phase Extraction of Organic Dyes  
To determine the adsorption of CR and MB from their aqueous solutions by the synthesized sorbent, we used the method described in [38]. The effect of pH of the sample solution, the amount of adsorbent, and the adsorption time on the removal efficiency was studied. The results are shown in Figure 4. In the initial period, the adsorption capacity of the sorbent increases rapidly, and then significantly decreases with increasing adsorption time. First, diffusion to the outer surface occurs, accompanied by diffusion into the pores  
Figure 3: N2 adsorption–desorption isotherms (**A**) and the pore size distribution curve (**B**) for MOF. Figure 2: Plausible structure of the sorbent. of the sorbent, because of which equilibrium is quickly achieved. The limiting adsorption, according to the experimental data, is 40 mg/g. The temperature dependence of the removal efficiency at pH = 7 is shown in Figure 5. It turned out that an increase in the temperature leads to a more rapid achievement of adsorption equilibrium and the limiting degree of removal is reached within 20 min. The resulting product of the SPE of CR and the synthesized sorbent luminesces in blue when irradiated with UV light with a wavelength of 385 nm, which may indicate that a \(\pi\)-\(\pi\) interaction occurs between the dye molecule and the adsorbent during the extraction process (Figure 6) [39]. Figure 4: Dependence of adsorption on time. Figure 5: Dependence of removal efficiency on time at different temperatures: (**A**)—CR, (**B**)—MB. Based on the results of the study, it can be concluded that the sorbent effectively removes the CR dye from an aqueous solution at a temperature of 35 \({}^{\circ}\)C. However, at a temperature of 18 \({}^{\circ}\)C, this complex was not effective enough. When extracting MB food coloring with a sorbent in an aqueous solution, the opposite situation is observed. A more efficient extraction is observed at a temperature of 18 \({}^{\circ}\)C with a short-term increase in the first 10 min, at which, apparently, the substance activation process occurs, since after 15 min, the concentration of the MB dye sharply decreased from 25.8 mg/L to 0.4 mg/L. Thus, after 15 min, the concentration of the dye decreased by 64.5 times, and after 60 min, by 7500 times, which indicates a high efficiency of the sorbent in the extraction of the MB dye at room temperature. When the dye is extracted at a temperature of 35 \({}^{\circ}\)C, the moment of activation is also present, but it is shifted and appears after 30 min. In the period from 30 min to 45 min, the concentration of the food coloring MB in an aqueous solution decreases from 16.8 mg/L to 4.4 mg/L, that is, almost four times. This indicates the effectiveness of using the sorbent, but not as effective as at room temperature. The study of the effect of acidity was carried out in the pH range 3-11. We have found that the degree of MB removal is practically independent of the pH of the medium in this range, while for CR, this dependence is quite significant (Figures 7 and 8). Figure 6: Appearance of the SPE product of CR: (**A**)—in daylight and (**B**)—in ultraviolet light (\(\lambda=385\) nm). Figure 7: Dependence of the dye removal efficiency on time at different pH (**A**) and on pH (**B**). To describe the adsorption of dyes using the synthesized sorbent, a pseudo first-order kinetic model was used, the compliance of which was confirmed by regression analysis using the least squares method (Figure 9). The pseudo first-order adsorption rate constants calculated by the graphical method are shown in Table 1.  
The Langmuir and Freundlich adsorption isotherms of dyes using a sorbent are shown in Figures 10 and 11, and Table 1 shows the coefficients of these isotherms. Most of the R\({}^{2}\) values are greater than 0.9 for all isotherm models. The R\({}_{\rm L}\) values for the Langmuir adsorption of the dye on the sorbent were between 0 and 1, indicating favorable adsorption.