Structural Evolution and Sorption Performance of Fly Ash-Based Geopolymers for Trivalent Actinide Sequestration

This study investigates the structural evolution and sorption performance of fly ash-based GPs (FA-GP) and GP modified with γ-Al ₂ ​O ₃ ​ (FA-Al-GP) for the sequestration of trivalent actinides, using ²⁴¹ Am(III). Comprehensive characterization via XRD, FTIR, and SEM-EDS confirmed that geopolymerization leads to the consumption of the amorphous FA phase, while ²⁷ Al MAS NMR revealed the successful integration of γ-Al ₂ O ₃ ​ into the aluminosilicate framework. Although BET analysis showed a reduction in surface area from 41 m ² /g (FA-GP) to 16 m ² /g (FA-Al-GP), the alumina-modified matrix exhibited superior densification and a more refined polymeric network. Sorption experiments demonstrated that both GPs achieve high uptake efficiencies (> 97%) across a wide pH range (4–8). Kinetic data obeyed the pseudo-second-order model, indicating a chemisorption-controlled process. Equilibrium sorption data were well fitted by the Langmuir isotherm, confirming monolayer adsorption on energetically favorable sites. FA-Al-GP exhibited significantly enhanced sorption capacities compared to FA-GP, reaching a maximum capacity of ~ 66 mg.g ^− 1 at 328 K, suggesting that γ -Al ₂ ​O ₃ modification introduces highly accessible, thermally stable active sites. Mechanistic insights were provided by Time-Resolved Fluorescence Spectroscopy (TRFS) and X-ray Photoelectron Spectroscopy (XPS). TRFS of sorbed Eu(III) revealed inner-sphere complexation with aluminol and silanol groups present in the GPs. XPS analysis further evidenced the interaction between trivalent ions and oxygen-donor groups (Si-O- and Al-O-) through distinct binding energy shifts in O 1s. These findings underscore that γ-Al ₂ ​O _{3 ​} modification not only enhances the structural integrity of FA-Al-GP but also significantly improves its capacity for radioactive waste remediation.