Valorization of solid waste incinerator fly ash by geopolymer production for removal of anionic bromocresol green dye from water: Kinetics, isotherms and thermodynamics studies

In this study, solid waste incinerator fly ash (SWI-FA) based geopolymer adsorbents were prepared by alkalination of hazardous SWI-FA and cured at different temperatures of 30 0C (GP30), 50 0C (GP50), 70 0C (GP70), and 90 0C (GP90). The geopolymers were applied as low-cost adsorbents for the removal of anionic bromocresol green (BCG) dye from water under varying conditions. The precursor and geopolymers were characterized by XRD, SEM-EDS, FT-IR, and point of zero charge (PZC). The geopolymers were morphologically different with varying chemical compositions. The kinetic data were best described by the pseudo second order model. The experimental equilibrium adsorption data were fitted to Langmuir, Freundlich, Temkin and Dubinin-Radushkevich-Kaganer adsorption isotherm models. The Langmuir monolayer maximum adsorption capacities increased consistently with curing temperature, from 41.70 mg/g to 515.5 mg/g for GP30 and GP90, respectively, a new benchmark for these materials for anionic dyes. The thermodynamic parameters, namely enthalpy (ΔH<0), Gibbs free energy (ΔG<0), entropy (ΔS >0) and activation energy (Ea) indicated that the processes are spontaneous, exothermic, physical (Ea<5kJ/mol and ΔH<40kJ/mol) and enthalpy-driven. The adsorption mechanisms included strong electrostatic interactions and hydrogen bonding. The production of these geopolymers provides dual advantage of waste valorization with applications in depollution of water.