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 synthesised from mixtures containing 40, 50 and 60% fly ash to the total solid mass and then activated by 8, 10 and 12 M sodium hydroxide (NaOH) solutions. Batch experiments were conducted for the removal of lead (Pb), lead (II), and zinc (Zn), zinc (II), from leachate at an ambient temperature of 24°C after a 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 lead (II) and zinc (II) occurred with the lowest percentage – that is, 60% – of clay in the synthesised geopolymers due to the diminished competitor ion concentration released by the alkali activation of clay. There, moreover, existed a notable correlation between the porosity and adsorption capacity of the adsorbents to develop the eminent adsorption of the heavy metals, particularly for lead (II) removal. The study of Langmuir and Freundlich isotherms also showed that the adsorption of lead (II) is described by homogeneous surfaces, while heterogeneous surfaces characterise zinc (II) removal. This refers to the different adsorption process for the removal of zinc (II).