## 3 Results and Discussion
### Cu2+ Immobilization by Geopolymers  
The temporal evolution of the dissolved Cu2+ concentrations for the experiments of the Cu:GP (without pH adjustment) and Cu:GP\({}_{\mathrm{pH}}\) (with alkaline treatment) series is illustrated in Figure 2. A fast decrease of the Cu2+ concentration was seen in all experiments within <60 min, which was followed by a slowly decreasing trend thereafter (Figure 2a,b) until chemical equilibrium conditions were attained after ~15 h, judged from the establishment of constant EC values until the experiments were terminated after 24 h. The Cu2+ removal was generally higher in the Cu:GP\({}_{\mathrm{pH}}\) series yielding final Cu2+ concentrations below the recommended value for Cu2+ in drinking water (<1.3 mg/L) [11]. Thus, the alkaline treatment resulted in high removal efficiencies for Cu2+ in the case of the Cu:GP\({}_{\mathrm{pH}}\) series (about 100%), whereas the Cu:GP series exhibited moderate to high Cu2+ removal efficiencies, ranging from 34 to 91% (Figure 2c,d). The fate of Cu2+ in the experiments of the Cu:GP series depended on the initial Cu:GP ratio used, i.e., the Cu2+ stock solutions had pH values between 3 and 5 before the reaction with GP and thus on the final solution pH, which varied between 4.1 and 5.7. This suggests that the reactivity of Cu2+ was largely pH dependent. This observation supports the conclusions drawn by [PERSON] et al. [52], who showed that strongly acidic conditions (pH 2) favor leaching and dissolution of Cu-bearing materials, such as Cu slags, whereas close to neutral (pH 7-8) and alkaline (pH 12-13) conditions enhance the stability of Cu-bearing minerals.  
Figure 2: Compilation of results from the Cu2+ immobilization experiments. (**a,b**) Evolution of the dissolved Cu2+ concentration for experiments of the Cu:GP series (left panel) and Cu:GP\({}_{\mathrm{pH}}\) (right panel) series; (**c,d**) Fraction of Cu2+ immobilized by GP with(out) alkaline treatment; (**e,f**) Cu2+ removal capacity of the Cu:GP series (left panel) and Cu:GP\({}_{\mathrm{pH}}\) series (right panel).  
The amount of Cu\({}^{2+}\) immobilized in the Cu:GP and Cu:GP\({}_{\text{PH}}\) series ranged from about 40 to 193 mg/g and 44 to 560 mg/g, respectively (Figure 2e,f). Higher Cu\({}^{2+}\) concentrations did not result in elevated Cu\({}^{2+}\) removal capacities, which indicates that the highest q\({}_{\text{e}}\) values present approximately the maximum removal capacity ('q\({}_{\text{m}}\)') of the GP mixture considered in this study. However, we note that co-precipitation of Cu minerals occurred in all experimental series, so that we cannot apply 'classical' adsorption isotherm models and pseudo-first-order or pseudo-second-order kinetic models to further fit our experimental data. The 'q\({}_{\text{m}}\)' value obtained for experiment Cu:GP-1:2 is 27% higher than Cu\({}^{2+}\) removal by an amorphous geopolymer synthesized from fly ash (152 mg/g at 45 \({}^{\circ}\)C) [27] and 79% higher than the amount of Cu\({}^{2+}\) removed from solution by an organically modified, metakaolin-based mesoporous geopolymer (108 mg/g at 30 \({}^{\circ}\)C) [53]. Moreover, Cu\({}^{2+}\) uptake by GPs with(out) alkaline conditioning was much higher than prior reported for other geopolymeric substances [54]. Experiment Cu:GP\({}_{\text{PH}}\)-1:2 yielded a q\({}_{\text{e}}\) value for Cu\({}^{2+}\) uptake in the same order of magnitude than previously reported for Pb\({}^{2+}\) uptake by porous geopolymer-based microspheres (629 mg/g at 25 \({}^{\circ}\)C) [55]. This comparison underlines the efficiency of GP for Cu\({}^{2+}\) uptake from solution.