In this research, the application of geopolymers as sustainable and efficient sorbent materials for the removal of methylene blue (MB) from synthetic aqueous solutions was investigated. Geopolymers were synthesized using pumice substituted with 10% and 15% alumina. Alkaline activator solutions consisting of sodium silicate and sodium hydroxide were used, with sodium hydroxide concentrations ranging from 1 M to 10 M. The materials with the best performance in MB removal were determined based on the MB batch removal tests, with the geopolymers synthesized using a 9 M concentration of NaOH showing the highest efficiency. The geopolymers and the precursor were characterized using various analytical techniques. X-ray diffraction analysis revealed the presence of new crystalline phases and an increase in amorphous regions within the geopolymers. SEM-EDS revealed morphological differences between the precursor, which had a smooth surface, and the geopolymers, which had rough surfaces due to the incorporation of aluminum. These findings were corroborated by the changes observed in the Fourier transform infrared spectroscopy analysis. Solid-state ²³Na MAS NMR revealed different sodium environments associated with crystalline domains and framework charge-balancing Na⁺ in the geopolymeric gel, as well as Na⁺ redistribution after MB adsorption, indicating partial ion exchange within the geopolymeric network. In addition, physisorption analysis confirmed the mesoporous nature of the geopolymers. The best conditions for MB removal using the geopolymer were pH 10, sorbent dosage of 0.2 g, initial MB concentration of 15 ppm, and contact time of 3 hours. Electrostatic interactions and mass transfer processes were found to exert a significant influence on the removal process. The kinetic and equilibrium data were modelled using the pseudo-first order, pseudo-second order, and Elovich models, along with the Langmuir, Freundlich, and Temkin isotherms. The analysis of the sorption parameters, kinetic models, and isotherms indicated adsorbate–adsorbent electrostatic interactions, consistent with the Na⁺ redistribution observed by solid-state NMR.
Santamaría et al. (Sun,) studied this question.