## 3 Results and Discussions
Given that the Fe\({}^{2+}\) ion does not act like an anion in this scenario, iron does not pose severe issues under the potential circumstances of the solution. However, because of its complexation with sulfate ions, the Zn\({}^{2+}\) ion behaves like an anion, in the form of ZnSO4\({}^{-2}\), and can contaminate the final samples. Nevertheless, since both ions behave as hydroxides at basic pH values, both impurities can be purged from the molybdenum solution during the elution stage. Figure 8a,b show the results of the continuous ion adsorption tests. Because of their affinity for anion\(-\)exchange resins and their abundance in solution, chloride ions occupy the bulk of anionic resin active sites. At low concentrations of chlorides in solution (1 g/L Cl\({}^{-}\)), the resins reached breaking points were 620 BV for the DOWEX M43 resin  
Figure 7: (**a**) Iron speciation for the copper solution. (**b**) Zinc speciation for the copper solution using the MEDUSA software. and 891 BV for the Lewatit MP 62 WS resin. However, as the concentration of chlorides in solution increases, for 5 g/L Cl\({}^{-}\), the breakpoints dropped to 250 BV for DOWEX resin and 405 BV for the Lewatit resin, and at high chloride concentrations (10 g/L), they dropped to 185 BV and 245 BV, respectively. These drops in the breakthrough points of the continuous ion exchange tests caused the original process to recover molybdenum to become economically unprofitable due to the high consumption of chemical reagents to carry out the subsequent molybdenum elution from the column. Figure 9 show the influence of the temperature on the continuous ion adsorption tests. The diffusion coefficient and the equilibrium conditions attained for the isotherms are both affected by temperature. Although working at room temperature is the most usual procedure in ion exchange, these two methods can boost molybdenum recovery yield. Though increasing this parameter can enhance the results, the rise in the breakthrough point from 890 BV at 25 \({}^{\circ}\)C to 907 BV at 40 \({}^{\circ}\)C and 940 BV at 55 \({}^{\circ}\)C was insufficient to justify the practical application. As a result, the process was carried out at room temperature. Finally, Figure 10 illustrates the elution curves for both resins using a 5% NaOH solution. Ionic species are usually eluted from the column by increasing the ionic strength of the buffer or by increasing the salt concentration or a pH gradient to gradually increase the species concentration. Given the low order of magnitude of the R\({}_{\text{L}}\) affinity coefficient, a high pH gradient is necessary to reverse the molybdenum desorption process from the ion exchange resin. In this case, the pH gradient from the acidic solution to the NaOH solution allowed effective molybdenum desorption. The pH of the elution solution also affects the molybdenum speciation and, therefore, the selectivity and the ionic strength of the buffer, decreasing its retention from the resin [53; 54]. Decreasing the chloride ions improves the resin's adsorption capacity, but it also increases the amount of the reagents required to recover molybate ion MoO\({}_{4}\)[2]- from the resin's surface. Thus, the molybdenum eluate solutions reached concentrations of 1000 mg/L for Lewatit resin and 890 mg/L for DOWEX resin. Table 5 illustrates the recovery efficiency of molybdenum and Fe and Zn impurities carried over during elution. These results show that Zn impurities can slightly contaminate these solutions, with a maximum of 0.32% for the Lewatit resin. If they were to accumulate as a result of multiple cycles, Zn may be a problem for the elution, but iron  
Figure 8: (**a**) Continuous ion exchange tests for DOWEX resin. (**b**) Continuous ion exchange tests for LEWATIT resin. tions are not transferred to the basic NaOH solution, so they do not contaminate the final molybdenum product. Figure 10: (**a**) Molybdenum elution curves for DOWEX resin. (**b**) Molybdenum elution curves for LEWATIT resin.