## 4 Conclusions  
In this study, we evaluated the metal adsorption efficiency of four cation exchange resins (PS--DVB, PA--DVB, TFSA, and OGL) from single-metal aqueous solutions, with a particular focus on Critical Raw Materials (CRMs) such as Cu\({}^{2+}\), Co\({}^{2+}\), Ni\({}^{2+}\), Ag\({}^{+}\), and Pb\({}^{2+}\). The investigation focused on removal kinetics and adsorption performance parameters. The results demonstrated that PS-DVB showed superior removal efficiency across all metals, with pseudo-second-order adsorption kinetics. PA-DVB's efficiency was more variable, achieving 85.44% removal for Ni\({}^{2+}\) in particular. The TFSA resin displayed an overall lower adsorption capacity but demonstrated specific selectivity for certain metals, especially Ni\({}^{2+}\) (84.86% removal). The lignin-derived OGL resin, despite showing limited overall removal capacity under experimental conditions, exhibited notable selectivity for Ni\({}^{2+}\), with a removal efficiency of 78.58%. This suggests potential applications in sustainable and selective Ni\({}^{2+}\) recovery processes, although further optimization studiesare warranted to enhance its broader applicability. Therefore, while conventional resins (PS--DVB and PA--DVB) remain highly effective for broad-spectrum metal removal in current industry, innovative materials like TFSA show potential for selective metal recovery applications, especially if further developed and engineered for specific applications. This selectivity could be particularly valuable for targeted CRM recovery in industrial processes. The results of this work are relevant to both water treatment and Critical Raw Material recovery. The proposed approach may provide valuable guidance for selecting appropriate materials for specific metal removal applications, contributing to both environmental protection and resource recovery objectives. Future research should focus on (i) investigating the performance of these ion exchange materials under competitive multi-metal conditions, (ii) developing enhanced Nafion-based and lignin-based matrix materials, and (iii) optimizing operational parameters for selective metal recovery. Additionally, studies on resin material regeneration, life cycle, and economic viability assessment for large-scale applications will be crucial for industrial implementation. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/separations11120357/s1](https://www.mdpi.com/article/10.3390/separations11120357/s1)]([https://www.mdpi.com/article/10.3390/separations11120357/s1](https://www.mdpi.com/article/10.3390/separations11120357/s1)), Figure S1: Time-dependent concentration of available Ag+ metal ions in bulk solution (ppm) examined by ICP-EOS; Table S1: Summary of kinetic model fitting results for various Ag+-resin pairs; Table S2: Experimental and theoretical adsorption results for Ag+-resin pairs. Figure S2: Morphological characterization of ion exchange resins: PS--DVB (A), PA--DVB (B), TFSA (C), and OGL (D). Conceptualization, L.A.H., S.B., A.P. and A.R. ; methodology, L.A.H., S.B., and A.P. ; validation, L.A.H., S.B., G.M., R.H. and A.R. ; formal analysis, L.A.H., S.B. and L.S. ; investigation, S.B., L.S. and L.A.H. ; resources, R.H., G.M. and A.P. ; data curation, S.B. ; writing--original draft preparation, S.B. ; writing--review and editing, all authors; supervision, A.P. and A.R. ; project administration, L.A.H. and A.R. ; funding acquisition, L.A.H. and A.R. All authors have read and agreed to the published version of the manuscript. The A.H. and S.B. acknowledge that this research was partially funded by the European Commission through the I4-GREEN Project, grant number 101084028. APC has been kindly waived by MDPI. Data Availability Statement: Data are contained within the article or Supplementary Material. The authors [PERSON] and [PERSON] were employed by the company Nanofaber S.r.l. These authors declare that Nanofaber S.r.l.