At a flow rate of 20 mL·s⁻¹, as illustrated in Figure 4C, the Li+ concentration presents a prolonged saturation phase between 10 and 50 minutes, followed by an abrupt drop to values below 1000 mg·L⁻¹ at the end of the cycle. This phenomenon suggests a late reorganization of the adsorption front or an increase in retention efficiency after the initial filling of surface sites. Regarding contaminants (Figure 4D), a sodium elution peak is noted at 20 minutes, reaching approximately 11 mg·L⁻¹, followed by a decrease to values near zero. Under the highest evaluated flow rate (30 mL·s⁻¹), the breakthrough curve profile becomes significantly steeper, reflecting the formation of a less efficient and more extensive Mass Transfer Zone (MTZ). The rapid passage of the solute through the fixed bed prevents the adsorption front from being fully processed, resulting in early ion leakage into the effluent. This is illustrated by the exponential increase in Li+ concentration in Figure 4E at the end of 60 minutes, reaching values near 14,000 mg·L⁻¹. Regarding other ions, the IRC 748 resin demonstrates robust selectivity for calcium, maintaining residual concentrations below 5 mg·L⁻¹ under most conditions, which confirms the efficacy of the chelating functionality in the purification of complex saline solutions. However, the sodium profile at 30 mL·s⁻¹ (Figure 4F) exhibits a concentration peak exceeding 20 mg·L⁻¹ in the first 15 minutes, followed by a gradual decrease. This "initial vulnerability" effect is characteristic of systems operating at high flow rates, where immediate ionic competition displaces sodium fractions weakly retained on the surface. In summary, for the IRC 748 resin, operation at lower flow rates optimizes the refining process, ensuring a more compact mass transfer zone and rational utilization of the adsorptive capacity, which ensures a lower mass of metal retained per gram of adsorbent relative to the treated volume. (A) (B) 113
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