Geopolymer-based adsorbents for pollutants removal

Geopolymers are intrinsically mesoporous and the micro-meso-macro-ultramacro porosity of the material can be tailored by adding fillers or using techniques able to induce porosity, as ice-templating or direct foaming. Furthermore, geopolymers possess ionic exchange and electrostatic interaction properties, thanks to the negative charge present on the final 3-dimensional structure, because of the presence of aluminum in tetrahedral coordination. Commonly, cations as sodium and potassium balance the geopolymer lattice, endowing the material with ionic exchange capacity, in analogy with zeolites, its crystalline counterpart. Therefore, the porous structure and the ionic exchange capacity make these materials interesting for applications in the field of adsorption. Indeed, these materials can be employed for the removal of pollutants in gas or liquid phase and, for this purpose, geopolymer-based adsorbents have been developed for the removal of CO2 or the treatment of wastewater, in particular for the removal of dyes. Composites containing zeolites or hydrotalcites were produced in monolith or granulate form for the adsorption of CO2 at different temperatures, while millimeter-sized beads were synthesized for the removal of dyes, exploiting different spherification processes (direct dripping in liquid nitrogen or polyethylene glycol or taking advance of the ionotropic gelation of Na-alginate).