Geopolymers are increasingly studied as alternative, cost-effective, environmentally friendly adsorbents for the removal of pollutants both in liquid and gaseous phase.
Geopolymerization is an eco-sustainable process based on the chemical reaction between an aluminosilicate powder and a highly alkaline solution: the reaction is in aqueous solution, with a low consolidation temperature (<80°C) and with the possibility to add fillers to improve the functionalities of the materials.
Playing on the initial composition (Si/Al molar ratio, type of alkaline solution and process conditions), it is possible to synthesize geopolymers with different structures and properties. Moreover, different types of zeolites can be directly nucleated from the geopolymer matrix.
Indeed, geopolymers are often compared to zeolites, especially with regard to their synthesis and final properties. As zeolites, geopolymers have ionic exchange and electrostatic interaction properties deriving from their peculiar 3-dimensional structure, because of the presence of aluminium in tetrahedral coordination. Furthermore, although geopolymers are amorphous due to the short-range ordering of their network, Al is in eight-membered or larger aluminosilicate rings as in synthetic zeolites.
Since zeolites are commonly used as adsorbents for the treatment of wastewater or for the removal of CO₂, geopolymers have the potential to be used as alternative in these fields of application, offering some advantages.
Geopolymers are obtained through simpler and less energy-intensive synthesis and can boast high mechanical performance with an easy shaping and reproducibility even on a large scale. Indeed, self-supporting, scale designable, porous and easy-to-handle materials, can be produced.
Mesoporous geopolymers offer the possibility to be shaped in different geometries with interconnected micro-meso-macro-ultramacro pores, enabling a fast mass transport and diffusion of cations/molecules towards the internal porosity.
For these reasons, different geopolymers and composites were produced as adsorbents and deeply characterized in terms of micro and macrostructure, porosity, chemical composition, mechanical resistance,..
In particular, geopolymer beads and granules were produced for the treatment of wastewater, and geopolymer-zeolite composites for the removal of CO₂ from gaseous streams.
Moreover, given the possibility to directly nucleate zeolite NaA starting from Na-based geopolymer matrix, in view of an alternative low-cost synthesis method to obtain zeolite NaA as a "solid" in a complex form, the total conversion of the matrix into zeolite was assessed also by addition of synthetic zeolite NaA as seed for nucleation.