## 1 Introduction
Selectivity for specific metals is often suboptimal, requiring multiple treatment cycles to achieve the desired purification standards. These limitations have spurred research into alternative approaches, focusing on innovative materials that can deliver superior performance while maintaining economic and environmental sustainability. Cation exchange resins modified with sulfonic groups show remarkable adsorption capacity and selectivity toward various cations present in aqueous solutions, making them valuable tools in water treatment [19]. The cation exchange process with resins functionalized with carboxylic groups plays a crucial role in determining ion exchange properties. The presence of these groups allows for widespread use in water treatment applications. carboxylic-group-loaded cation exchange resins have proven effective in capturing metals such as copper (Cu), iron (Fe), and manganese (Mn) under controlled pH conditions [20]. In this context, exploring and benchmarking alternative ion exchange materials is essential. This study covered four material options. The first material was Nafion, the brand name for a popular ionomer consisting of sulfonated tetrafluoroethylene-based fluoropolymer-copolymer. While not an obvious choice from a sustainability standpoint, the novelty lies in its second life reuse: the Nafion included in this study was recovered and recycled at ENEA and Nanofaber through physical methods [21], demonstrating an innovative approach to valorize the sophisticated and expensive material within a circular economy framework. This represents a significant advancement in the sustainable use of advanced polymers for engineering applications. The high cation exchange capacity and ionic conductivity make Nafion ideal for water purification applications [22; 23]. Nafion's chemical structure, featuring highly hydrated sulfonic groups, facilitates rapid interactions with metal cations in wastewater. This polymer can form cationic complexes, enabling the adsorption of heavy metals such as Ni(II), Co(II), Pb(II), Cu(II), and Ag(I) [24]. The second candidate included in this study was lignin, a natural and highly sustainable biopolymer. It is not only an abundant industrial by-product, but also offers unique chemical properties that can be tailored for enhanced performance. Its use as a metal getter represents a novel and ambitious choice for the application at hand, aligning strongly with the principles of circular economy. Lignin's abundant availability and its intrinsic environmental sustainability make it an impactful biomaterial in the transition toward greener technologies. Lignin's chemical structure, rich in functional groups such as phenolic, hydroxyl, and carbonyl groups, promotes various interactions--ion exchange, complexation, and electrostatic interactions--that are essential for heavy metal adsorption [25; 26]. In particular, lignin fractionated via the Organosolv process possesses a heterogeneous surface chemistry with a high density of functional groups that can attract highly charged metal ions [27]. The ion exchange process in lignin involves the substitution of mobile hydrogen ions from the carboxylic and phenolic groups with metal cations [28], enabling effective heavy metal removal from wastewater. A critical advantage of lignin is its versatility, as it can undergo chemical modifications, such as grafting functional groups or forming composites with other materials, to significantly enhance its adsorption capacity and selectivity for specific metal ions [29]. These transformations not only improve lignin's performance, but also expand its applicability, positioning it as a highly adaptable and promising alternative to conventional, less sustainable ion exchange resins. Moreover, its biocompatibility and environmental friendliness further reinforce its appeal as a green material for advanced water treatment technologies. The first and second cation exchange materials in this preliminary study are reference systems represented by commercially available engineered resins.