Adsorptive dye removal using clay-based geopolymer: Effect of activation conditions on geopolymerization and removal efficiency

Clay-based adsorbents offer a promising approach to addressing wastewater contamination. This research is aimed to study metakaolin-based geopolymer adsorbent for treating methylene blue dye from aqueous systems. A variation in the Na₂SiO₃/NaOH content to get a SiO₂/Na₂O ratio in the activator solution ranging from 1.3 to 1.5, and a further variation in metakaolin to alkali activator is done to prepare different batches of geopolymer adsorbent. Here, the SiO₂/Al₂O₃ molar ratio varies from 2.84 to 3.52. The effect of these ratio variations on the geopolymerization and its adsorption capacity is studied. Raw clay and the prepared adsorbent were characterized by different techniques to confirm the chemical composition, phases formed, surface area, and morphology. The adsorbents are used to conduct a batch adsorption study on cationic dyes using UV–visible spectroscopy. The adsorption study confirmed that cationic dye-contaminated water can be treated using the prepared adsorbents with selected batchs having a removal efficiency of more than 80% for methylene blue dye with optimised operating conditions. Electrostatic interactions initiate the adsorption of cationic dyes onto the geopolymer surface, while chemisorption reinforces and stabilizes the process, with both mechanisms working synergistically to enhance adsorption efficiency. The batch adsorption data for methylene blue followed the pseudo-second-order kinetic model and the Langmuir isotherm model, indicating monolayer chemisorption.