Facile synthesis of geopolymer composites for effectual removal of Cr(VI) and Cd(II) ions from aqueous solutions

Aquatic ecosystems face significant degradation due to both natural and anthropogenic pollution, leading to the release of considerable quantities of hazardous substances from several point as well as non-point source worldwide. The large amounts of fly ash generated annually offer potential for recycling as low-cost adsorbents, particularly for the removal of heavy metal ions. This study focuses on investigating the adsorption mechanisms of Cd(II) and Cr(VI) ions on fly ash-based geopolymer composites. Batch adsorption experiments demonstrated significant adsorption capacities of 18.75 mg/g for Cd(II) at pH 4.5 and 17.47 mg/g for Cr(VI) at pH 1.5, using 0.1 g of adsorbent over a contact time of 90 min. The Langmuir isotherm model provided the best fit for Cr(VI) adsorption, while the Freundlich isotherm model was most suitable for Cd(II), suggesting multilayer adsorption in the latter case. Adsorption kinetics were best described by pseudo-second-order model. Thermodynamic analysis revealed that the adsorption of Cr(VI) was an exothermic process, whereas Cd(II) adsorption was endothermic; both processes were spontaneous and thermodynamically feasible, as indicated by the calculated entropy, enthalpy, and Gibbs free energy values. Unlike traditional adsorbents, the synthesized geopolymer composite having the ability to target multiple ions or performs well in the challenging aqueous environment. The geopolymer composite is a low cost, easy synthesis, requires less energy, or uses environmentally benign materials. This research provided a promising adsorption approach of heavy metals removal from landfill leachate.