## 3 Results and Discussion
### Implications for Water Treatment and Environmental Remediation  
Ion exchange and leaching reactions had only a minimal impact on the mineralogical, chemical and microstructural composition of the Cu-GPs obtained from the immobilization experiments with(out) alkaline treatment (Figures 4 and 5). This indicates that the vast majority of the Cu\({}^{2+}\) ions was bound to 'unreactive', barely soluble mineral phases, such as gerhardtite under acidic conditions versus tenorite and spertinite under alkaline conditions (Figure 3a,c), in addition to the Cu\({}^{2+}\)-substituted in the GP matrix. The low reactivity of all Cu\({}^{2+}\)-containing phases under the herein tested conditions suggests that a high immobilization degree of Cu\({}^{2+}\) was achieved, i.e., only \(\leq\)2.4% of the Cu\({}^{2+}\) bound within the Cu-GPs was remobilized (Figure 4b,e and Figure 5a,c). This highlights the potential of granulated GP for water treatment applications, especially in acidic aqueous media where sorbent materials must withstand corrosive conditions [15; 16; 20]. Moreover, GP can be easily removed by filtration or sedimentation from treated wastewater, ensuring economic use of this novel sorbent material (Figure 6a-c). Figure 5: Compilation of results obtained from the leaching experiments. (**a,c**) Fraction of Cu\({}^{2+}\) liberated to the solutions upon acid leaching in the Cu:GP and Cu:GP\({}_{\text{pH}}\) series; (**b,d**) SE images of the leached samples Cu:GP-1:2 and Cu:GP\({}_{\text{pH}}\)-1:2 revealing no microstructural changes. [PERSON] et al. [65] argue that metal ion (Me\({}^{2+}\))-bearing C-A-S-H, a common constituent of Ordinary Portland cement, will rapidly decompose even under mildly acidic conditions, thereby providing Ca\({}^{2+}\) and Me\({}^{2+}\) ions to the aqueous phase and leaving least soluble solid residues, such as amorphous silica and poorly crystalline aluminosilicates, at the leaching front. This dissolution reaction could thus release harmful Cu\({}^{2+}\) ions to the aquatic environments if no other immobilization mechanisms are active. Alike, the leaching behavior of geopolymers is known to increase with acidity and especially at pH < 4.5, the structural integrity of the K-A-S-H network becomes increasingly weakened [52]. Nevertheless, our results suggest a low reachability of Cu\({}^{2+}\) from the Cu-GPs under mildly acidic conditions (Figure 5), which indicates that chemical bonds are responsible for the binding of Cu\({}^{2+}\) to the molecular structure of the GPs [66]. However, their long-term resistance to corrosion in highly acidic environments (pH < 4) has to be demonstrated, but this is beyond the scope of this study. The type of binding of Cu\({}^{2+}\) ions in the three-dimensional molecular network of geopolymers is key for predicting their stability in corrosive systems. The behavior of Cu\({}^{2+}\)-containing low-Ca geopolymers is often viewed analogously to zeolite minerals [67], because of their apparent crystal-chemical similarities. Zeolites are scaffold silicates, consisting of cross-linked (Si,Al)O\({}_{4}\) tetrahedrons with Na\({}^{+}\) or K\({}^{+}\) ions occupying exchangeable sites to keep electrostatic neutrality. Natural and synthetic zeolites can immobilize various pollutants (e.g., radioactive components and heavy metal ions) through ion exchange [15]. Our results indicate that the Cu-GPs had a high resistance against Na\({}^{+}\) exchange (Figure 4), suggesting that the exchange processes with other Me ions are also limited. On the other hand, the porous geopolymer network may allow the sorption and/or incorporation of potentially other hazardous Me cations from solution given that these dissolved components have an identical ionic charge and a Me ionic radius similar to Cu\({}^{2+}\), such as Co\({}^{2+}\), Mn\({}^{2+}\), nickel (Ni\({}^{2+}\)), palladium (Pd\({}^{2+}\)) and Zn\({}^{2+}\)[67].