Rapid Adsorption of Cationic Methylene Blue Dye onto Volcanic Ash-metakaolin Based Geopolymers

In this study, four geopolymer sorbents GP0, GP10, GP30 and GP50 were synthesized using volcanic ash (VA) and metakaolin (MK) blends as precursors with 0, 10, 30 and 50% MK content by mass, respectively. The materials were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy, and Brunauer–Emmett–Teller (BET) surface area analyses, revealing successful geopolymerization of the precursors and increasing surface area with increasing MK content. The sorption performance of the VA, MK and VA-MK geopolymers was then evaluated for the removal of cationic methylene blue (MB) dye from aqueous media. Sorption capacity was independent of composition, providing flexibility in sorbent synthesis. Sorption rate, on the other hand, was 3–8 times greater for the VA-MK geopolymers than the precursor materials. The equilibrium adsorption data were suitably explained by the Freundlich model, denoting multilayer adsorption onto a heterogeneous adsorption surface with higher Freundlich affinity constant (KF) for geopolymers than VA. The adsorption kinetics obeyed the pseudo-second-order (PSO) kinetic law with an average of 98% removal efficiency in 30 min. MB uptake was pH-dependent and driven by electrostatic chemisorption interactions. These results motivate further studies on the use of locally sourced geopolymers for water purification applications.