Radioiodine Sorption on AgCl-modified Bentonite and its Stability in different Environments

This study investigated the stability and radioiodine (I ⁻) sorption properties of AgCl-modified bentonite sorbents under conditions simulating engineered safety barriers (concrete-bentonite contact) in a deep geological radioactive waste disposal facility (GDF).

Synthetic groundwater from the «Yeniseisky» site was filtered through Portland Cement Concrete (PCC) and High Alumina Concrete (HAC) samples. Filtrate pH/Eh were measured, and chemical composition was analyzed via ICP-MS. AgCl-bentonite sorbents were synthesized using two methods (AgCl HMTA and AgCl HYD). Stability was assessed by monitoring Ag dissolution in filtrates using Volhard’s method. Iodide sorption was evaluated using natural and modified bentonite.

PCC filtrate showed higher pH (12.43) and lower Eh (−74 mV) than HAC filtrate (pH 11.10, Eh +6 mV). PCC contained elevated Ca²⁺, while HAC contained trace phosphorus. No Ag dissolution occurred from sorbents in filtrates, confirming stability. AgCl-bentonite exhibited significantly higher I ⁻ sorption (K_d = 373±87 – 1070±230 mL·g⁻¹) than natural bentonite (K _d = 64±28 mL·g ⁻¹), with rapid equilibrium (1 hour).

The results demonstrate that AgCl-modified bentonite retains high stability and exceptional I ⁻sorption capacity even in alkaline, concrete-impacted environments relevant to GDFs. The absence of Ag dissolution underscores its suitability for long-term containment. However, phosphorus in HAC filtrates may pose a risk of Ag₃ PO ₄ formation, warranting further study.

AgCl-modified bentonite is a promising engineered barrier material for radioiodine isolation in GDFs, particularly at the «Yeniseisky» site. Its efficacy persists in concrete-contact scenarios, though phosphorus interactions require additional investigation.