2554 capacity and a final value of 66.04 mg·g⁻¹. This value surpasses the 53.14 mg·g⁻¹ recorded for the IRC748 resin under the same experimental conditions. This behavior indicates that, although both resins exhibit affinity for alkali metals, the structure of IRC747 kinetically favors lithium retention in solutions with high initial concentration (6,997 mg·L⁻¹). On the other hand, the IRC748 resin shows higher operational selectivity when the objective is to avoid excessive lithium removal during the purification stage. Compatible results were reported by Valles (2024), who investigated the behavior of lithium ions on IRC747 resin and observed only discrete reductions in lithium concentration over time, without compromising the viability of the brine purification process. The authors also indicate that, in this system, the rate-limiting step of the ion exchange process is predominantly associated with intraparticle diffusion—that is, the migration of the ion through the polymeric matrix of the resin (HÉRÈS et al., 2018). Regarding sodium (Na⁺) removal, the IRC747 resin exhibited an exchange capacity of 139.26 mg·g⁻¹ within 60 minutes, while the IRC748 resin demonstrated superior performance, reaching an exchange capacity (qt) of 157.28 mg·g⁻¹ in the same period. These results indicate greater efficiency of the IRC748 resin in removing sodium from the solution, promoting a more significant reduction in the concentration of this contaminant. The observed performance difference between the resins may be associated with the initial ionic composition of IRC747, which contains Na⁺ ions in its structure, as indicated in Table 1. This characteristic may have partially limited the additional exchange capacity for this cation, reducing the efficiency of the adsorption process due to the lower ion exchange gradient available in the system. With respect to divalent ions, both resins exhibit marginal loading capacities for calcium (Ca²⁺), with values stabilizing at approximately 0.08 mg·g⁻¹ after about 15 minutes of contact. The rapid establishment of equilibrium indicates that the low calcium concentration in the solution (4.2 mg·L⁻¹) constitutes the main limiting factor of the process, resulting in immediate saturation of the sites available for this ion. For magnesium (Mg²⁺), the exchange capacity remains zero for both resins, as the residual concentration (Ct) remains identical to the initial value (0.001 mg·L⁻¹), evidencing the absence of significant interaction or limitations in analytical detection for extremely low concentrations. The kinetic behavior observed for sodium (Na⁺) on the IRC748 resin shows qt values with reduced temporal variation between 15 and 60 minutes, signaling that the system rapidly approached adsorption equilibrium conditions. IRC748 thus exhibits a greater thermodynamic affinity for sodium, achieving an exchange capacity of 157.28 mg·g⁻¹. This outcome may be linked to faster surface kinetics or structural features of the polymeric matrix, such as pore distribution and reduced mass transport resistance for this particular cation. The stability observed in sodium values over time indicates that the rate-limiting step occurs predominantly in the initial moments of contact between the solution and the adsorbent material, with a regime close to system equilibrium subsequently established. The work of Foureaux (2021) found similar behaviors for Amberlite IRC 747 resin in the removal of cobalt, nickel, and copper ions. This corroborates that, in this context, the distribution of species within and on the surface of the resins exerts a direct influence on the performance of the ion exchange process, such that the adsorption capacity tends to increase according to the quantity of active sites available on the surface.
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