## 1 Introduction  
Heavy metals, such as lead (Pb), copper (Cu), cadmium (Cd) and mercury (Hg), are hazardous environmental pollutants in Earth's surface settings, considering their negative impact on human health and ecosystems [1; 2; 3; 4; 5]. Most heavy metals are persistent, non-biodegradable and tend to cumulate in, e.g., soils, sediments, surface water, groundwater and living organisms, causing disorders and diseases [6; 7; 8; 9]. Although being an essential microelement and dietary element to plants, animals and humans, Cu compounds are often used as antimicrobial agents to prevent microbial growth on materials' surfaces, e.g., as coatings in roofing, sheathing and plumbing. However, at higher concentrations, i.e., \(>\)5-10 mg Cu\({}^{2+}\)/day in food [10] or \(>\)1.3 mg/L Cu\({}^{2+}\) in drinking water [11], Cu\({}^{2+}\) possesses strong bisostatic properties and a high toxicity, leading to Cu\({}^{2+}\) poisoning in severe cases [12]. Consequently, in several studies, Cu is proposed to be added to the European Watch List of emerging substances due both to its toxicity potential and increasing abundance in aqueous media, soils and sediments [13; 14]. Thus, there is a high demand for the development of efficient, low-cost and eco-friendly ('green') cleaning agents in order to minimize the harmful effects of Cu\({}^{2+}\) in aquatic media [15]. A large number of techniques can be used for the removal of Cu\({}^{2+}\) from aqueous media, such as adsorption [16], chemical precipitation [17], bioremediation [18], electrokinetic processes [19] and ion exchange [20], etc. These water treatment technologies greatly differ in, e.g., efficiency, operational costs, material and resource uses, waste production and sustainability aspects. So far, only a few attempts have been made to sequester dissolved Cu\({}^{2+}\) directly in secondary minerals that possess a high affinity towards Cu\({}^{2+}\) uptake, such as Cu-chelates [21], Fe-(hydr)oxides [22], Cu-(oxy)hydrate chlorides and Cu-carbonates [23; 24], Cu-sulfate hydrates [25], calcium-aluminum-silicate-hydrates (C-A-S-H) [26] and geopolymers [27; 28]. In particular, the latter bear a high potential for heavy metal immobilization [29; 30; 31; 32] through chemical immobilization. Geopolymers, when formed using low-calcium (low-Ca) aluminosilicates, are inorganic binders that form three-dimensional, non-crystalline aluminosilicate structures with exchangeable sodium (Na\({}^{+}\)) or potassium (K\({}^{+}\)) ions balancing the negative charges arising from the tetrahedral substitutions of aluminum (Al\({}^{3+}\)) for silicon (Si\({}^{4+}\)) [33]. They are typically formed by reacting metakaolin and/or other inorganic materials having pozzolanic properties, such as fly ash and calcinated clay, with water glass and alkali hydroxide solutions [15]. Their interconnected, micro- to nano-porous structure is advantageous for (ad)sorption processes and metal ion incorporation [31]. Further, geopolymers possess an excellent resistance to temperature [34; 35] and chemical corrosion [36; 37], rending their application in environmental remediation possible. For example, geopolymers have been successfully utilized for Cu\({}^{2+}\)[31], Pb\({}^{2+}\), zinc (Zn\({}^{2+}\)), Cd\({}^{2+}\)[38; 39], manganese (Mn\({}^{2+}\)) and cobalt (Co\({}^{2+}\)) [29] removal from aquatic media, both by chemical precipitation and structural incorporation. In this study, the immobilization of Cu\({}^{2+}\) ions by granulated, metakaolin-based geopolymer, as well as their mineralogical and chemical properties and subsequent resistance to ion exchange and leaching, were investigated using a set of equilibrium-approaching batch experiments run under variable pH conditions at ambient temperature. The efficiency and the main reaction mechanisms underlying Cu\({}^{2+}\) uptake are assessed using a multi-methodological approach. Implications for water treatment and environmental remediation are discussed.