Interplay between geopolymer formulation, microstructure, and strontium sorption properties

• Decreasing Si/Al ratio decreases porosity, while enhancing sorption properties. • Silicon environment plays a significant role in the sorption properties. • Q 4 (4Al) and Q 4 (2Al) are keys sorption promoters to enhance sorption properties. • A specific microstructure is necessary to give access to all exchange sites. • A non-monotonic selectivity toward strontium is observed. This article focused on developing chemically stable, high-performance adsorbents for selective Sr 2+ removal from nuclear wastewater, enabling safer long-term storage and disposal. Geopolymers, due to the negative charge of aluminum acting as a sorption exchange site (Al IV ), promote ion exchange, thereby exhibiting favourable adsorption kinetics and exchange capacities. A novel approach is developed to comprehensively characterize geopolymers and establish structure-sorption property relationships. This study examines how key synthesis factors, specifically the Si/Al and H 2 O/M 2 O molar ratios, influence structural properties, characterized by nitrogen adsorption–desorption and 27 Al and 29 Si Nuclear Magnetic Resonance (NMR). These techniques probe material structural properties and Al IV concentration, enabling assessment of their impact on strontium and calcium sorption behaviour. Increasing the Si/Al ratio enhances porosity but reduces Al IV concentration, thereby diminishing sorption capacity. This decline is attributed to greater distances between Al IV units and a structural shift toward a more silicon-dominated network. A 2D surface state model based on the Q 4 (mAl) silicon centers from the 29 Si NMR was developed to correlate it to the sorption properties of the geopolymer. Selectivity toward the strontium is observed for Si/Al = 1.52, highlining the impact of the porous structure and silicon environment, with favourable distance between Al IV sites for strontium sorption over calcium. Similarly, increasing the H 2 O/K 2 O ratio improves sorption properties. However, once a specific ratio is reached (H 2 O/K 2 O = 12), microstructure no longer influence sorption properties. Silicon environment differ across compositions, influencing selectivity factor·H 2 O/K 2 O = 12, exhibiting the highest one, indicating a more suitable sorption environment.