## 1 Introduction  
The development of energetically efficient wastewater treatment technologies for the removal of small organic pollutants or heavy metal ions is of paramount importance for maintaining a clean environment and mitigating current water pollution problems. The advantages and drawbacks of various existing technologies for metal ion removal from wastewater, such as chemical precipitation, ion exchange, adsorption, membrane filtration, coagulation and flocculation, flotation, electrochemical treatment, etc., have been extensively reviewed [1]. Finding alternative technologies to the currently existing methods that avoid secondary pollution, generation of solid waste, are energetically efficient and have low operation costs could be achieved by deployment of advanced materials, and this represents a forthcoming challenge for fundamental science and engineering. In this context, for extraction and removal of metal ion pollutants or hydrological mining of noble metals, liquid membranes technologies such as bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs) [2], supported liquid membrane (SLMs) [3] supported ionic liquid membranes (SILM) [4], or polymer inclusion membranes (PIM) [5, 6], nanoparticles incorporated into an absorptive film [7], have been proposed as energetically efficient alternatives to classical methods. All these alternative methods involve the interfacial transfer of the metal ions from the water phase into a liquid organic phase or a solid-state polymer containing chelating agents. For example, in ELMs methods, the oil-in-water (o/w) emulsion, with the organic phase containing a chelating agent, is stirred in a reservoir until the ion extraction is completed. For a large interfacial area and efficient extraction, small emulsion droplets are required, however, droplet coalescence and emulsion stability are the limiting factors of the method, which must be resolved [8]. One possible solution to solve the emulsion stability in ELMs is to use the more stable Pickering emulsions, which are emulsions stabilized by nanoparticles or amphiphilic Janus nanoparticles (JNPs) [9; 10]. In this case, amphiphilic nanoparticles such as JNPs can play a double role, as emulsion stabilizers in ELMs but also as ligand carriers, whereas ligands can be immobilized on the surface of the nanoparticle by specific surface grafting techniques. In this work we synthesize amphiphilic JNPs, which carry ligands and are also interfacially active, being capable of partitioning at the oil-water interface and are thus able to stabilize Pickering emulsions. JNPs are asymmetric nanoparticles consisting of at least two lobes differing in their chemical composition or surface properties. The contrasting properties between lobes give rise to an intrinsic amphiphilic property resembling molecular surfactants [11; 12]. With the help of these ligand-carrying amphiphilic JNPs, we emulsify molten paraffin wax to create o/w emulsions. The proof of concept of this technology is illustrated in the cartoon of Scheme 1. By emulsifying the molten wax in water, colloidosomes are obtained, which have a monolayer of amphiphilic JNPs at their surface. Amphiphilicity is key to the emulsification ability of the JNPs [13]. Colloidosomes can be defined as microcapsules with a shell of nanoparticles, which are obtained by self-assembly of nanoparticles at the interface between two immiscible liquids, most commonly water and oil [14]; these colloidosomes can be employed directly for ion extraction. Although the molten wax colloidosomes cannot preserve their integrity in ion extraction and recovery cycles, upon cooling, the liquid molten paraffin droplets solidify, generating surface nanostructured microspheres by trapping the Janus nanoparticles at the interface. The aim of this work is to show that water-floating wax microspheres decorated with ligand-carrying JNPs, which result from the solidification of wax-in-water Pickering emulsions, can be successfully employed for metal ion extraction, as depicted in Scheme 1.