## 3 Results and Discussions
### Metal Removal Efficiency and Resin Selectivity  
Figure 2 shows the removal trends of four metal ions (Cu\({}^{2+}\), Pb\({}^{2+}\), Co\({}^{2+}\), and Ni\({}^{2+}\)) using the four different cation exchange resins over a contact period of 40 min, with the results for Ag\({}^{+}\) available in the Supplementary Materials. Looking at the Cu\({}^{2+}\) in Figure 2, we can observe an initial Cu\({}^{2+}\) concentration of approximately 600 ppm across all resins. PS--DVB proved to be the most effective resin, reaching concentrations of approximately 200 ppm in the first 5 min and achieving near-complete removal (<50 ppm) within 40 min. PA--DVB showed moderate efficiency, reducing the concentration to 230 ppm by the end of the treatment. TFSA exhibited similar but slightly better performance compared to PA--DVB, reaching approximately 400 ppm. OGL demonstrated the poorest performance, maintaining consistently high concentrations throughout the treatment period. In relation to Pb\({}^{2+}\), we started with an initial solution concentration at 4000 ppm. PS--DVB demonstrated effective removal, reaching a concentration of 150 ppm in 40 min. The removal achieved by PA--DVB was good but more gradual, reaching 850 ppm. TFSA plateaued after 5 min around 2500 ppm, while OGL showed minimal removal capacity, maintaining concentrations close to the initial levels. In the case of Co\({}^{2+}\), the individual performance was quite limited, except for the PS--DVB resin, which showed good adsorption within 5 min and remained constant up to 40 min. The removal pattern for Ni\({}^{2+}\) showed more interesting features. Starting from solutions at concentrations of 1200 ppm, all resins showed a rapid initial removal within the first 10 min. PS--DVB and TFSA reached similar final concentrations of 170 and 176 ppm, respectively, while PA--DVB showed some concentration fluctuations before stabilization. Nafion has proven to be an excellent material for water treatment due to its unique chemical structure, characterized by the presence of polar groups such as sulfonic acids. These functional groups not only enable specific interactions with metal cations, but also provide the material with exceptional water and ion management capabilities. The study conducted by [PERSON] et al. (2018) offers valuable insights into the crucial role of sulfonic groups in ionic interactions. These functional groups are pivotal for cation exchange activity, as they attract metal cations through strong electrostatic interactions. The research highlights how Nafion promotes the formation of hydrophilic clusters that facilitate ion  
Figure 2: Time-dependent concentration of the available metal ions (Cu\({}^{2+}\), Pb\({}^{2+}\), Co\({}^{2+}\) and Ni\({}^{2+}\)) in bulk solution (ppm) examined by ICP-EOS. Each panel shows the remaining concentration of a specific metal ion over time (0, 5, 10, 20, and 40 min): (**A**) Cu\({}^{2+}\) concentration in bulk, (**B**) Pb\({}^{2+}\) concentration in bulk, (**C**) Co\({}^{2+}\) concentration in bulk, and (**D**) Ni\({}^{2+}\) concentration in bulk. transport. Although the study primarily focuses on proton transport, it can be inferred that metal cations, due to their positive charge, are effectively captured by the material's sulfonic groups. Nafion's ability to modulate its degree of hydration and the internal distribution of water is an additional factor that enhances both the mobility and adsorption of metals. Unlike Nafion, OGL has a completely different chemical structure, which results in significantly distinct behavior. Lignin is a heterogeneous and branched polymer, rich in hydrophobic aromatic units and functional groups such as methoxyl and hydroxyl groups, which limit its compatibility with aqueous environments. This inherent hydrophobicity can hinder its interaction with water, a crucial factor in systems relying on efficient cation exchange. Consequently, lignin's reduced hydration capacity may compromise the formation of hydrophilic domains necessary for ion mobility and adsorption.