## 3 Nanocomposite Catalysts  
### In Situ Synthesis Approach  
The successful application of core-shell-type anion exchange resins for the recovery of Au from WEEE required a series of optimizations, involving equilibrium studies on adsorption of Au(III), Pt(VI), and Pd(II) from synthetic solutions in 0.1-3 mol L\({}^{-1}\) HCl. Surprisingly, at lower concentrations of HCl, after adsorption of Au(III), the amino-functionalized resins changed their color to purple instead of the expected orange (see Figure 1C for reference). The phenomenon is displayed in Figure 4. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**System** & **Polymeric Base** & **Active Sites** & **Re(VII) Uptake** & **Comments** & **Ref.** \\ \hline Re + U & Ambserp & & & & \\ A920U & Strong base & 37 mg g\({}^{-1}\) & & Possible Re and U & \\  & A920U & Strong base & 37 mg g\({}^{-1}\) & separation & \\ \hline Re + As + Cu + Fe + Si & ZS70 & Weak base & 3.15 mg g\({}^{-1}\) & Effective Re separation & [32] \\ \hline Re & \multicolumn{4}{c}{Quaternized poly(4-vinyl)pyridine-co-divinylbenzene} & 61 mg g\({}^{-1}\) & Up to 170 mg g\({}^{-1}\) Re(VII) uptake during column studies} & [33] \\ \cline{2-5}  & Reliex 436 & Strong base & 46 mg g\({}^{-1}\) & Separation of Re and Mo & \\ \hline Re + Mo & Purolite A170 & Weak base & 167 mg g\({}^{-1}\) & \begin{tabular}{c} Separation of Re and Mo \\ almost impossible. \\ \end{tabular} & [34] \\ \hline Re + Mo & S-co-DVB & 4-amino-1,2,4-triazol & 354 mg g\({}^{-1}\) & \begin{tabular}{c} 17-fold greater affinity \\ toward Re(VII) when \\ compared to Mo(VI) \\ \end{tabular} & [35] \\ \hline Re + Mo & Merrifield resin & N-methylimidazole & 462 mg g\({}^{-1}\) & \begin{tabular}{c} Resin reveals affinity \\ toward Mo(VI) \\ \end{tabular} & [36] \\ \hline Re + Mo + V + Cu & VBC-co-DVB & 1-(3-aminopropyl)imidazole & 303 mg g\({}^{-1}\) & \begin{tabular}{c} Resins obtained \\ applying microwave \\ heat. Affinity toward \\ Re(VII) 200-fold greater \\ when compared to \\ Mo(VI). No adsorption \\ of V and Cu observed \\ \end{tabular} & [27] * \\ \hline \multicolumn{5}{c}{**Au(III) uptake**} \\ \hline Au & Purolute A500 Plus & Strong base & 17 g dm\({}^{-3}\) & \begin{tabular}{c} Possible 97\% of Au \\ recovery from a resin \\ \end{tabular} & [43] \\ \hline Au & Sericin and alginate crosslinked particles & 137–648 mg g\({}^{-1}\) & \begin{tabular}{c} 78–84\% recovery, using \\ thiourea in HCl \\ \end{tabular} & [42] \\ \hline Au cyanide & \begin{tabular}{c} Amberlite \\ leachate of PCB \\ \end{tabular} & Strong base & 11 mg g\({}^{-1}\) & \begin{tabular}{c} Up to 100\% recovery of \\ Au, using thiourea \\ stripping agent \\ \end{tabular} & [44] \\ \hline Au + Pb & \multicolumn{4}{c}{AmberliteXAD/THP} & 14 mg g\({}^{-1}\) & \begin{tabular}{c} Reduction-coupled \\ recovery of Au from \\ Pb-based stripping agent \\ \end{tabular} & [45] \\ \hline Au aqua regia & \multirow{2}{*}{\begin{tabular}{c} Amberlite XAD-4 \\ (S-co-DVB) \\ \end{tabular} } & \
\begin{tabular}{c} 1-(2-aminoethyl)piperazine \\ loaded on the core–shell \\ structure \\ \end{tabular} & 20 mg g\({}^{-1}\) &
\begin{tabular}{c} Resins selective toward \\ trace-quantities of Au, \\ resistant to aqua regia, \\ 100\% Au recovery \\ \end{tabular} & [41] * \\ \hline \hline \end{tabular}  
* Works of the authors.  
\end{table} 
Table 2: Polymeric adsorbents for the removal of Re and Au [27]. To explain this phenomenon, it was necessary to take a closer look at the adsorption of Au(III), as well as other noble metals (Pt(VI), Pd(II)) on the synthesized materials. Hence, a series of tests on adsorption capacity and elution of anion exchange resins bearing functionalities derived from 1,2-diamonoethane and 1-(2-aminoethyl)piperazine were tested in the batch tests on the uptake of the noble metals [47].