Geopolymer-hydroxyapatite composite foams for wastewater remediation

Two types of monolithic geopolymer-hydroxyapatite (HA) foams, different in terms of porosity, were produced by direct foaming through the addition of metallic silicon as blowing agent. The ability of the geopolymer to bind HA allowed an easy shaping and consolidation of the material. The total porosity of the two foams ranged from 72 to 87 % with similar surface areas (26–27 m2 g−1) despite different HA content (27 and 21 wt% respectively). Composite foams were compared with the pure geopolymer matrix assessing the surface electrostatic characteristics and testing methylene blue (MB) removal at pH = 7. At long times (24h) the best performing monolithic foam reached a removal efficiency of 76 % and 69 % respectively in 10 ppm and 50 ppm MB solution, while granulated foams reached an efficiency of 85 %. The best experimental data fit with the pseudo-second-order kinetic model indicating the chemisorption of MB.