Geopolymer-hydrotalcite hybrid beads by ionotropic gelation

Geopolymer beads are already known as alternative, cost-effective, environmentally friendly adsorbents for cationic species in wastewater treatment. To broaden the spectrum of applications it is necessary to functionalize the geopolymer matrix with fillers and create composites. In this study, being hydrotalcite an anionic exchanger, highly reproducible geopolymer-hydrotalcite hybrid beads were synthesised. Starting from a metakaolin-based geopolymer slurry added with a sodium alginate solution and hydrotalcite powder as filler, millimetre-sized beads were shaped by ionotropic gelation using injection-solidification in CaCl2 solution. In order to vary the porosity and related properties of the beads, two consolidation methods were adopted: a conventional consolidation in a heater set at 60 °C and a freeze-drying process. Beads differed in terms of dimension and morphology with an open porosity ranging from 50 to 70% and specific surface area from 12 to 23 m2 g-1. Mechanical resistance, following ISO 18591, was about 6 MPa making the beads easy to handle and resistant in the recovery, separation and filtration operations of aqueous systems.