Determining role of surface chemistry and textural architecture in methylene blue adsorption and hydrogen storage in sepiolite

Abstract

In this study, natural sepiolite and APTES-functionalized sepiolite were investigated as multifunctional clay-based materials for aqueous-phase methylene blue (MB) removal and gas-phase hydrogen storage. The effects of surface chemistry and pore architecture on adsorption performance were systematically evaluated through FTIR, XRD, BET, SEM/EDX, TGA/DTG, and zeta potential analyses. The results showed that APTES functionalization preserved the fibrous structure of sepiolite while reducing micropore accessibility and specific surface area. Electrokinetic measurements revealed a shift of the isoelectric point toward more acidic values and the predominance of negative surface charge over a wide pH range, enhancing electrostatic interactions with cationic MB molecules. Consequently, the modified sample exhibited improved liquid-phase adsorption performance. Adsorption capacity and adsorption rate increased with increasing stirring speed, initial dye concentration, pH, and temperature, whereas adsorption was limited under acidic conditions and low stirring rates. Kinetic modeling indicated that the adsorption process was best described by the pseudo-second-order model and followed a multi-step mechanism involving film diffusion, intraparticle diffusion, and surface interactions. Hydrogen storage experiments demonstrated that adsorption was predominantly governed by physisorption and strongly influenced by microporous structure. Natural sepiolite exhibited higher hydrogen uptake than the modified sample due to its greater micropore accessibility. Overall, liquid-phase adsorption was controlled primarily by surface chemistry and electrokinetic properties, whereas gas-phase hydrogen storage was dictated by pore architecture. These findings provide insights into the development of multifunctional clay-based materials for environmental remediation and hydrogen-storage applications.