Immobilisation of iodide in alkali-activated materials

At the Fukushima Daiichi Nuclear Power Station (FDNPS), continuous water circulation cools fuel debris, leading to the presence of radionuclides such as Sr-30, Cs-137, and I-129 in the cooling water. These radionuclides are adsorbed and co-precipitated by various materials. Among them, I-129 is a key radionuclide for safety assessment during the final disposal of adsorbent and co-precipitation materials, owing to its long half-life and poor sorption. Alkali-activated materials (AAM) have been demonstrated to immobilise cationic radionuclides but not anionic radionuclides. Here, layered double hydroxide (LDH), magnesium oxide (MgO), and silver nitrate (AgNO₃) were investigated as additive materials to immobilise I^- in potassium-based AAM (K-AAM). Our findings revealed that LDH and MgO were not effective in retaining I^- in K-AAM. In the case of LDH addition, the high anion exchange capacity of LDH was rendered ineffective, as its adsorption sites were occupied by highly selective silicate anions. In the case of MgO addition, magnesium silicate hydrates (M-S-H) were formed, which did not appear to possess the ability to retain I^-, despite the desirable in situ synthesis of LDH containing alumina in K-AAM. Conversely, the addition of AgNO₃ to K-AAM resulted in a reduced leaching rate, which was attributed to AgI formation. The immobilisation could be enhanced by adjusting the amount of AgNO₃ incorporated into the K-AAM during the fabrication process. In conclusion, the leaching of I^- from spent adsorbent and co-precipitation materials embedded in the K-AAM matrix could be suppressed when AgI was sufficiently generated within the K-AAM matrix.