Structural evolution and enhanced sorption performance of γ-Al₂O₃-modified fly ash-based geopolymers for trivalent...

This study evaluates pristine fly ash-based geopolymers (FA-GP) and γ-Al₂O₃-modified geopolymers (FA-Al-GP) as low-cost sorbents for trivalent actinide and lanthanide sequestration, utilizing radiotracer ²⁴¹Am(III) and Eu(III) as a chemical surrogate. Structural characterization confirmed the successful framework integration of γ-Al₂O₃. While this modification reduced the physical surface area from 41 m²/g (FA-GP) to 16 m²/g (FA-Al-GP), confirmed by Brunauer-Emmett-Teller-specific surface area analysis method, it produced a denser matrix with a higher proportion of octahedrally coordinated aluminum suggested by solid-state ²⁷Al magic-angle spinning nuclear magnetic resonance. Consequently, FA-Al-GP exhibited superior macroscopic sorption, achieving  >97% uptake across a broad pH range (4.0-8.0) and a higher maximum capacity (~ 66 mg·g⁻¹ at 328 K) compared to pristine FA-GP (~ 39 mg·g⁻¹). Sorption for both matrices followed pseudo-second-order kinetics and Langmuir isotherms, indicating spontaneous, endothermic monolayer chemisorption. Spectroscopic analyses (time-resolved fluorescence spectroscopy (TRFS) and X-ray photoelectron spectroscopy (XPS)) provided molecular-level evidence that uptake is driven by strong inner-sphere complexation with reactive aluminol and silanol groups. These findings demonstrate that γ-Al₂O₃ modification fundamentally enhances the sorption capacity by concentrating highly accessible, Lewis-basic active sites, proving that surface chemistry dictates capacity more than bulk physical surface area.