## 2 Synthetic Protocols for Hydrometallurgy
Taking into account the obtained results, the anion exchange resin, based on the enhanced VBC-co-DVB copolymer modified in MV-field, revealed great adsorption capacity (303 mg Re g\({}^{-1}\)), but, most important, the resin revealed a 200-fold greater selectivity toward Re(VII), as compared to excess of Mo(VI), V(V), and Cu(II). This allowed for proposing a solution for the hydrometallurgical challenge of separating Re from Mo [31; 32; 33; 34; 35; 36]. The activity involving new IX and chelating materials for hydrometallurgy was also aimed on the resins for the recovery of Au from wasted electric and electronic equipment (WEEE). Within the carried-out research, we have received a series of _aqua regia_ leachates of electronic scrap (CPUs and pin-contact elements), characterized by the trace amount of Au (4 ppm) and over 1000-fold excess of competitive metals (such as Ni, Zn, Cu, etc.). The small concentrations of Au required its quantitative extraction from WEEE to a leachate, hence, it was done using _aqua regia_, as alternatives were much more harmful (cyanides) or much less efficient (thiourea) [37; 38; 39]. All of this created a serious challenge, as the anion exchange resins designed for such a media should reveal, on the one hand, great selectivity toward Au(III) ions, and on the other hand, they should be resistant to the degradation by _aqua regia_. For this purpose, we developed a new type of amino-functionalized anion exchange resins, revealing a core-shell-like structure. The synthetic protocol, as displayed in Figure 3, involved impregnation of highly crosslinked macroporous Amberlite XAD-4 adsorbent with VBC and DVB monomers further polymerized to create a layer of VBC-co-DVB copolymer functionalized in the next step, using aliphatic and heterocyclic amines. Crosslinked polymeric base (XAD-4) ensured chemical stability of the material, while the localization of the functionalities in its outer layer allowed us to  
Figure 2: Structure of anion exchange resin obtained in MV heat and simplified mechanism of IX of Re(VII) ions. Figure reproduced from Reference [27], with permission of Elsevier. reduce the time of the adsorption process [40]. What is more, selectivity of the resin was ensured by incorporation of 1-(2-aminoethyl)piperazine, revealing a strong affinity toward Au(III) [26; 41]. As the result, the anion exchange resin was not only resistant to _aqua regia_ but also allowed to receive 100% recovery of Au from WEEE. The materials synthesized for the selective separation and recovery of Re and Au are set together with others reported in the literature and displayed in Table 2. Based on these, the proposed materials revealed competitive adsorption capacity, selectivity, and durability in the challenging processes of the separation and recovery of the precious metals. Based on the literature review (see Table 2 for reference), it can be deduced that the proposed anion exchange resins allow us to resolve challenges related to the separation of Re(VII) and Mo(VI), that are particularly important when considering processing Mo-Cu ores, which are the main source of Re. As compared to the literature, the IX resins obtained in MV field led to the emittance of the suppression of ReO\({}_{4}\)- binding ions [34], as well as the necessity of strict control of pH [35], providing at the same time a tempting anion exchange capacity of 303 mg Re g\({}^{-1}\)[27]. On the other hand, the proposed core-shell-type IX resins turned out to be resistant to _aqua regia_, allowing us to exclude problematic cyanide leaching of secondary raw materials [37], including possible and effective separation and recovery of Au from WEEE, where Au exists in ultra-trace amounts. Although the literature provides examples of more efficient separation materials [42], it must be noticed that the proposed resins are based on crosslinked suspension copolymers that provide a technological edge as compared to alternative bio-based polymers [42] and enable the use of the most efficient leaching approaches.