Mechanistic insights into radium adsorption on montmorillonite: DFT and experimental studies

Radium (Ra) released from spent nuclear fuel poses a challenge due to its long half-life (1600 years) and radiological hazard, but its rarity limits experimental studies. This study explores the Ra adsorption mechanism on montmorillonite, a key bentonite component in engineered barriers, using theoretical calculations and experiments. Theoretical results identified three stable Ra-complexes in aqueous solutions with different water and hydroxyl coordination numbers. Our findings demonstrate that Ra(OH)₂(H₂O)₄ forms inner-sphere complexes on (0 0 1) surfaces, exhibiting greater stability than Ra²⁺ and [Ra(H₂O)₈]²⁺ species. On the (0 1 0) surface, both Ra²⁺ and Ra(OH)₂(H₂O)₄ form inner-sphere complexes with stronger adsorption compared to [Ra(H₂O)₈]²⁺, which adsorbs through weaker hydrogen bonding. Additionally, Ra(OH)₂(H₂O)₄ adsorbed on the (0 1 0) surface showed the most stable form on the deprotonated surface at high pH. The stability of Ra adsorption in montmorillonite was also effectively demonstrated by charge transfer and density-of-states analysis. Furthermore, Ra batch adsorption experiments confirmed that Ra adsorption followed a linear isotherm, and pH-dependent experiments showed that Ra uptake was highest at high pH conditions, in agreement with theoretical results. These observations suggest that engineered barriers may mitigate Ra transport in deep disposal repositories subject to high pH induced by concrete and rock.