## 3 Results and Discussions
### Cation Exchange Capacity
These treatments allow for the introduction of new functional groups and improve the accessibility of existing ones, thus enhancing its adsorbent properties. For example, in the work by [PERSON] et al. [37], amine groups were introduced through the Mannich reaction, resulting in a significant improvement in copper ion chelation and adsorption capacity. For lead ions, [PERSON] et al. [38] added -CSS groups, increasing the complexation capacity with this metal. Another example is the work by [PERSON] et al. [39], which produced a doubly modified lignin with -CSS and -COOH groups, creating a highly effective material for lead adsorption. In [PERSON] et al. [40], Kraft lignin was combined with chitin, after modification with a 15% hydrogen peroxide solution. This combination produced a material enriched with available functional sites, such as phenolic OH and -COOH groups, thus increasing the number of active sites on the surface. Regarding cobalt sequestration, [PERSON] et al. [41] used Kraft lignin subjected to aminomethylation for the introduction of amine groups, useful for subsequent interactions with silica. This composite material demonstrated the coordination and adsorption capacity of cobalt ions in aqueous solution three times higher than the starting materials. In conclusion, the results obtained from this study demonstrate the effectiveness of various cation exchange resins in treating contaminated water and recovering critical materials. Traditional resins such as PS--DVB and PA--DVB, with their high capacity for removing heavy metals, are particularly well-suited for established industrial applications, such as the treatment of industrial and mining wastewater. These resins provide an immediately implementable option in existing treatment systems, enhancing efficiency and ensuring compliance with environmental regulations. The innovative material TFSA, based on recycled Nafion, has shown good removal capabilities for metals such as Cu, Pb, Ni\({}^{2+}\), and Ag\({}^{+}\), although with a generally lower performance compared to commercial resins. Nevertheless, TFSA represents a sustainable and technologically advanced alternative, particularly interesting for the selective recovery of critical metals and for applications where sustainability is a key factor. OGL, derived from lignin, offers an ecological and renewable solution, with particular effectiveness for nickel, although it requires further optimization to achieve performance comparable to traditional resins. It is important to emphasize, however, that any chemical modification of lignin to enhance its properties, such as the introduction of -CSS functional groups through the use of carbon disulfide (CS\({}_{2}\)), must be approached with utmost responsibility. CS\({}_{2}\) is highly toxic, and its use poses a significant risk to human health and the environment. For this reason, the development of lignin-based materials must adopt an approach that balances innovation and sustainability, promoting eco-friendly and low-impact processes. These innovative materials pave the way for the next generation of ion exchange resins, combining high performance, environmental sustainability, and targeted applications. As such, the recovery of Critical Raw Materials (CRMs) from industrial wastewater would not only reduce dependence on primary sources, but would also contribute to mitigating the risks associated with global supply chains, while promoting more sustainable water treatment technologies.