## 3 Results and Discussions
### Cation Exchange Capacity
Data for the OGL resin were not available for any of the considered metals, making it impossible to perform a comparative analysis of its performance relative to the other resins under the tested conditions. The examination of our experimental results indicates that the PS--DVB and PA--DVB resins exhibited (expectedly) high ion exchange capacities against the tested metals, with PS--DVB showing superiority in this panel, both in terms of removal effectiveness and accuracy of the inferred kinetic models. On the other hand, the TFSA resin boasted a mix of interesting performances and significant limitations in applications requiring efficient ion exchange. Lastly, OGL performed well vs. Ni\({}^{2+}\), which represents a novel observation and cannot be compared to any prior experimental data in the literature. The inferiority of the novel materials (i.e., TFSA-Nafion and OGL-lignin) did not come as a surprise in a comparison against highly optimized materials. Even though the PS--DVB resin provided an excellent short-term option in demanding water applications, such as the mining industry, our results also clearly demonstrated the ion exchange properties of TFSA-Nafion and OGL-lignin, which can be further engineered to formulate more sustainable next-generation resins that can bridge the gap with the current filtering technology in terms of performance and recyclability. Nafion is an monomeric polymer widely used in various industrial fields and recently investigated for applications in water treatment. In the literature, Nafion is primarily employed in the energy sector, specifically in fuel cells and hydrogen electrolyzer production, thanks to its exceptional proton conduction properties. However, alternative applications are emerging, including the sequestration of metal cations from aqueous solutions. [PERSON] et al. (2018) conducted a study on the recovery of the same metal cations studied by us using Nafion(tm) 117, demonstrating the material's effective behaviour in capturing such species. The results highlight that most of the sequestration occurs in the first few minutes of exposure, with a plateau reached within 30 min; specifically, the most significant absorption was recorded within the first 15 min. Similarly, in our experiments, we observed that the primary absorption occurred even more rapidly, in some cases within 5 min, with complete saturation reached around 30 min. [PERSON]'s study also showed a strong affinity of Nafion for various metal cations, with a particular preference for Ni\({}^{2+}\), a result also confirmed in our experiments. Another relevant aspect is that the experimentation conducted at room temperature produced results comparable to those obtained by [PERSON] in a temperature range between 25 \({}^{\circ}\)C and 65 \({}^{\circ}\)C, confirming that temperature does not significantly influence sequestration performance. A further promising application area for Nafion is the recovery of heavy metals from water through electrodialysis, using the material in membrane form rather than resin, in the presence of an electric field. The work by [PERSON] et al. (2018) demonstrated how Nafion, integrated into an electrodialysis system, allows for the separation and recovery of Ni\({}^{2+}\) and Co\({}^{2+}\) ions from sulphate-containing solutions. In this context, the focus was not on permanent ion sequestration, but on their temporary capture and subsequent efficient transfer through the membrane during the electrodialysis process. This study highlights Nafion's high affinity for the analyzed metals, further supporting the idea that it could be a highly promising material for the separation and treatment of ionic species. Unlike Nafion, lignin offers a significantly broader range of applications. However, compared to our study, in which we used untreated lignin to evaluate its absorption performance in its natural form, the scientific literature shows a preference for the chemical treatment of lignin.