Acid-activated geopolymers from red mud and metakaolin for methylene blue adsorption

• Industrial waste transformed into safe, low‑cost sorbent materials • Acid-activated red mud geopolymers show high methylene blue adsorption • Geopolymers from waste meet safety standards after repeated washing cycles • Adsorption–Diffusion Intraparticle Model best fits adsorption kinetics • Langmuir isotherm indicates monolayer adsorption behavior In this study, new acid-activated geopolymers were synthesized from red mud and metakaolin and, for the first time, evaluated as adsorbents for methylene blue (MB) removal from water. Their performance and leaching behavior were compared with conventional alkali-activated analogues, too. Leaching tests showed high chemical stability in aqueous solution for acid-activated materials: in particular, after four washing cycles, all wash waters satisfied drinking-water limits for the monitored species, supporting the suitability for environmental applications in aqueous media for the developed geopolymers. Batch adsorption experiments (C₀, MB = 50 mg L -1 ; 4 mg mL -1 solid loading) demonstrated that the best acid-activated sample achieved the highest equilibrium capacity ( q max = 27 mg/g) and was well described by the Langmuir isotherm, indicating monolayer physisorption. Alkali-activated geopolymers, instead, exhibited faster kinetics, reaching ∼95% MB removal within 5 h. Kinetic data for representative samples were described by an Adsorption–Diffusion Intraparticle Model (ADIM) with excellent agreement ( R 2 = 0.986), yielding effective surface diffusivities of ∼7 × 10 -14 m 2 ·s -1 (alkali-activated) and ∼3 × 10 -14 m 2 ·s -1 (acid-activated). These results support the use of waste-derived geopolymers as viable, cost-effective and environmentally friendly adsorbents for dye-contaminated wastewater treatment and define a practical trade-off: acid activation improves long-term uptake and leach stability, whereas alkaline activation enables rapid sorption processes.