## 3 Results and Discussions
### Cation Exchange Capacity  
The experimental and theoretical adsorption results for the different metal-resin combinations are reported in Table 2, which compares the experimental value of equilibrium adsorption capacity (_qe exp._), the calculated value (_qe calc._), the associated percentage error, and the metal removal percentage from the solution.The PS--DVB resin showed excellent removal capacities for all the tested metals, with _qe exp._ values very close to those predicted by kinetic models, as indicated by the low percentage errors. In the case of Cu\({}^{2+}\), it achieved a _qe exp._ of 184.85 mg/g with an error of 6.07% and a removal rate of 98.8%, demonstrating particular effectiveness. The performance was also remarkable for Pb\({}^{2+}\), with a capacity of 1994.8 mg/g, a minimal error of 0.26%, and a removal rate of 96.49%. For Ni\({}^{2+}\), PS--DVB maintained a good result, showing very low _qe_ errors (5.99%) and a removal percentage equal to 66.02%. Only for Co\({}^{2+}\) did the removal rate reduce to 51.18%, though still maintained a low _qe_ error of 1.93%. Overall, this resin demonstrated exceptional removal capacity for almost all the tested metals. The PA--DVB resin showed varying performance depending on the metal ion in solution. For Cu\({}^{2+}\), the _qe exp._ (109.64 mg/g) was lower than that of PS--DVB, with a relatively high error of 44.78% and a removal rate of 61.32%. For Pb\({}^{2+}\), it obtained a _qe exp_ of 1642.31 mg/g, an error of 2.85%, and a removal rate of 79.44%, showing good efficiency, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Metal–Resin** & _qe calc._ (mg/g) & _qe exp._ (mg/g) & **Error _qe_ (\%)** & **Removal (\%)** \\ \hline Cu\({}^{++}\)–PS—DVB & 196.07 & 184.85 & 6.07 & 98.8 \\ Cu\({}^{++}\)—PA—DVB & 158.73 & 109.64 & 44.78 & 61.32 \\ Cu\({}^{++}\)—TPSA & 74.11 & 68.78 & 7.75 & 36.47 \\ Cu\({}^{++}\)—OGL & — & — & — & 7.05 \\ \hline Pb\({}^{++}\)–PS—DVB & 2000 & 1994.8 & 0.26 & 96.49 \\ Pb\({}^{++}\)–PA—DVB & 1595.5 & 1642.31 & 2.85 & 79.44 \\ Pb\({}^{++}\)–TFSA & 833.33 & 797.31 & 4.52 & 38.57 \\ Pb\({}^{++}\)—OGL & — & — & — & 6.16 \\ \hline Co\({}^{++}\)–PS—DVB & 833.33 & 817.57 & 1.93 & 51.18 \\ Co\({}^{++}\)—PA—DVB & 704.04 & 485.07 & 45.14 & 30.36 \\ Co\({}^{++}\)—TFSA & 181.82 & 179.41 & 1.34 & 10.78 \\ Co\({}^{++}\)—OGL & — & — & — & — \\ \hline Ni\({}^{++}\)–PS—DVB & 434.78 & 462.48 & 5.99 & 66.02 \\ Ni\({}^{++}\)—PA—DVB & 704.04 & 598.48 & 17.64 & 85.44 \\ Ni\({}^{++}\)–TFSA & — & — & — & 84.86 \\ Ni\({}^{++}\)—OGL & — & — & — & 78.58 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental and theoretical adsorption results for metal–resin pairs. For each metal (Cu\({}^{2+}\), Pb\({}^{2+}\), Co\({}^{2+}\), Ni\({}^{2+}\)) and resin (PS—DVB, PA—DVB, TFSA, OGL), the table shows the calculated qe (_q calc._) value (equilibrium adsorption capacity in mg/g), the experimental qe (_q exp._) value, the percentage error of qe (_error qe_), and the metal removal percentage from the solution. Blank entries indicate the absence of available data or low affinity of the resin for the specific metal. The table allows for a comparison of adsorption efficiency and the accuracy of kinetic models for each metal–resin combination. though lower than PS--DVB. For Co\({}^{2+}\) and Ni\({}^{2+}\), the removal rates were 30.36% and 85.44%, respectively, suggesting variable selectivity for different metals. The TFSA resin generally showed lower removal capacities compared to the previous resins for almost all the tested metal ions. For Cu\({}^{2+}\), it reached a _qe exp_ of 68.78 mg/g, an error of 7.75%, and a removal rate of 36.47%, indicating low affinity. Pb\({}^{2+}\) removal was also reduced, with a _qe exp_ of 797.31 mg/g, an error of 4.52%, and a removal rate of 38.57%. For Co\({}^{2+}\), it showed a low _qe exp_ of 179.41 mg/g, an error of 1.34%, and a removal rate of 10.78%. The lack of data for Ni\({}^{2+}\)further limited the overall assessment.