2555 Figure 03: Ion exchange capacity of IRC747 and IRC748 resins for Li and Na. Source: Authors. 3.2 Batch removal of contaminants The experimental data presented in Figure 4A-B demonstrates significant differences in the resin's exchange capacity regarding the various cations present in the solution. A higher affinity for monovalent ions (Figure 4A and C) is observed, particularly for sodium and lithium, compared to the divalent calcium and magnesium ions (Figure 4B and D). This trend is corroborated by the loading capacity values at time (qt), which, after 60 minutes of contact, reach 139.26 mg·g⁻¹ for sodium and 66.04 mg·g⁻¹ for lithium, evidencing higher retention efficiency for monovalent cations under the evaluated experimental conditions. The removal kinetics for sodium and lithium ions exhibit a profile of continuous decrease in solution concentrations over the 60-minute interval, indicating that the thermodynamic equilibrium of the system was not fully established during the monitored period. The high initial concentration of these ions establishes a significant driving force, favoring the diffusion of analytes from the solution to the active sites of the resin. The IRC747 resin shows a more pronounced lithium removal compared to the IRC748 resin. In the latter, the variation in lithium concentration remained practically constant. A similar behavior was observed for the sodium ion, whose concentration remained virtually unchanged, evidencing the absence of significant removal by this resin during the experimental period. Regarding the behavior of the calcium ion in the IRC747 resin (Figure 4B), concentration variations between 0.3 and 0.53 mg·L⁻¹ were observed, indicating the occurrence of simultaneous processes of removal and subsequent release of the ion back into the solution. Throughout the experiment, values below 0.28 mg·L⁻¹ were recorded, suggesting fluctuations associated with the establishment of the ion exchange equilibrium in the system. Conversely, in the IRC748 resin (Figure 4D), calcium removal occurred more efficiently, starting with a concentration reduction from approximately 0.70 mg·L⁻¹ to 0.25 mg·L⁻¹, corresponding to a removal of approximately 35.71%. These results indicate the superior performance of the IRC748 resin in calcium removal compared to the IRC747 resin under the same experimental conditions. For magnesium (Figure 4B and D), the recorded exchange capacity was practically zero for both resins, as the residual concentration (Ct) remained equivalent to the initial concentration (0.001 mg·L⁻¹). This behavior can be attributed to two main factors: the extremely low concentration of the ion in solution, possibly below the detection limit of the analytical method employed, or the low selectivity of the resins for magnesium in the presence of high concentrations of competing ions, such as sodium and lithium, which tend to preferentially occupy the available active sites in the ion exchange matrix. The analysis of the batch assays indicates that the resin exhibits high selectivity for alkali cations, with loading capacities following the preference order Na+ > Li+ >> Ca2+ > Mg2+, evidencing greater exchange efficiency for monovalent ions compared to divalent ions under the evaluated experimental conditions (XIAN ZHANG et al., 2019).
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