## 1 Introduction  
Anthropogenic activity causes the direct and indirect discharge of billions of contaminants of different natures and origins [1; 2; 3; 4; 5], which need specific treatment for their removal [6; 7; 8; 9]. Potential toxic metals (PTMs) pollution comes from different sources--such as batteries, metal products, and industrial chemical discharge [10; 11]--and generates adverse health effects. The presence of PTMs in the environment is increasing in some parts of the world, especially in emerging countries [12; 13; 14; 15; 16].  
In recent years, cheap and eco-friendly sorbents [17; 18; 19] have been used to remove PTMs from wastewater. These sorbents may be considered an alternative to conventional activated carbon--which has a high cost of production and regeneration [20; 21]--or ion exchange resins, which are expensive and pH-dependent [22]. Compared to conventional methods, the use of these cheap materials allows for \"once-through operations\" (i.e., PTM-laden, or exhausted, sorbents can be put into a cement conglomerate as aggregate) [17; 23; 24; 25; 26], thus overcoming the expensive regeneration procedures and problems related to the large, hazardous sludge production that is typical of traditional sorbents.  
In this paper, a silico-aluminate sorbent, such as expanded perlite, was used to remove lead ions from wastewater. Lead ion was chosen, because it was considered an important pollutant, as it is very toxic to mammals and aquatic life, with its presence in the environment the result of various applications [27; 28]. The typical concentration of lead ion in industrial wastewater is between 5-15 mg\(\cdot\)L\({}^{-1}\) for battery production, while for fertilizers, mining, and metallic industries, it is approximately 2 mg\(\cdot\)L\({}^{-1}\); accordingly, in this study, synthetic lead ion concentration was chosen in a range comparable to industrial practice (2-10 mg\(\cdot\)L\({}^{-1}\)) [29; 30]. Perlite is a vitreous, volcanic rock that, if heated over 870 \({}^{\circ}\)C, expands up to 20 times its original volume, leading to the formation of an excellent thermal insulating and ultra-lightweight material, characterized by interesting mechanical and chemical resistances [31]. For these reasons, and due to its relatively low cost (40-80 USD per m\({}^{-3}\)), perlite has recently been suggested for many applications, regarding wastewater treatment (i.e., for the synthesis of filtration membranes; as a carrier in biological, fluidized beds; as support for TiO\({}_{2}\), and as a sorbent for Cr(III) ions and dyes) [31; 32; 33; 34; 35]. Additionally, the correct adoption of perlite for wastewater treatment may be considered a sustainable solution, as it is in agreement with most of the twelve principles of green chemistry [36].  
In this study, packed bed columns were prepared and lead ion separation tests carried out at room temperature and in distilled water, with different sorbent dosages, influent concentrations, and flow-rates being used. The analysis was carried out by evaluating the thermodynamic and kinetic aspects, together with the microstructural characterization, of perlite.  
Exhausted perlite residues were reused as cheap and eco-friendly aggregates to produce construction materials--such as cement conglomerates--thus overcoming problems associated with the cost of regeneration. PTM-laden beads can be encapsulated by a matrix, minimizing the leaching of toxic compounds, and are thermal insulating materials with good mechanical resistances [17; 23; 24]. In this respect, conglomerates with increasing perlite volume and with different specific weights were prepared and characterized by thermal mechanical tests.  
Consequently, besides proposing a sustainable wastewater treatment, the reuse of exhausted, expanded perlite to increase the insulating properties of cement is in accordance with circular economy principles [37] and with the recent trend towards sustainable water treatments [38; 39; 40].