Clay–fly ash geopolymer characterisation and application for the removal of lead and zinc

Abstract

This research aims to enhance the adsorption capacity of local clay using a fly ash-based geopolymer representing active-passive liner materials. Clay-fly ash geopolymers were synthesized from the mixtures containing 40, 50 and 60% fly ash to the total solid mass and then activated by 8, 10 and 12 M NaOH solutions. Batch experiments were conducted for the removal of lead, Pb(II), and zinc, Zn(II), from leachate at ambient temperatures of 24°C after 90 min contact time. Then, for the clay-fly ash geopolymer exhibiting the best adsorption capacity, the effects of contact time, initial concentration of solutes and curing time on its adsorption capacity were thoroughly studied. The best performance for the adsorption of Pb(II) and Zn(II) occurred with the lowest percentage, i.e. 60%, of clay in the synthesized geopolymers due to the diminished competitor ions concentration released by the alkali activation of clay. There, moreover, existed a notable correlation between porosity and adsorption capacity of the adsorbents to develop the eminent adsorption of the heavy metals, particularly for Pb(II) removal. The study of Langmuir and Freundlich isotherms also showed that the adsorption of Pb(II) is described by homogenous surfaces, while the heterogeneous surfaces characterize Zn(II) removal. That refers to the different adsorption process for the removal of Zn(II).