Evaluation of materials for iodine and technetium immobilization through sorption and redox-driven processes

Abstract

Radioactive iodine-129 (¹²⁹I) and technetium-99 (⁹⁹Tc) pose a risk to groundwater due to their long half-lives, toxicity, and high environmental mobility. Based on literature reviewed in Moore et al. (2019) and Pearce et al. (2019), natural and engineered materials, including iron oxides, low-solubility sulfides, tin-based materials, bismuth-based materials, organoclays, and metal organic frameworks, were tested for potential use as a deployed technology for the treatment of ¹²⁹I and ⁹⁹Tc to reduce environmental mobility. Materials were evaluated with metrics including capacity for IO₃^- and TcO₄^- uptake, selectivity and long-term immobilization potential. Batch testing was used to determine IO₃^- and TcO₄^- sorption under aerobic conditions for each material in synthetic groundwater at different solution to solid ratios. Material association with IO₃^- and TcO₄^- was spatially resolved using scanning electron microscopy and X-ray microprobe mapping. The potential for redox reactions was assessed using X-ray absorption near edge structure spectroscopy. Of the materials tested, bismuth oxy(hydroxide) and ferrihydrite performed the best for IO₃^-. The commercial Purolite A530E anion-exchange resin outperformed all materials in its sorption capacity for TcO₄^-. Tin-based materials had high capacity for TcO₄^-, but immobilized TcO₄^- via reductive precipitation. Bismuth-based materials had high capacity for TcO₄^-, though slightly lower than the tin-based materials, but did not immobilize TcO₄^- by a redox-drive process, mitigating potential negative re-oxidation effects over longer time periods under oxic conditions. Cationic metal organic frameworks and polymer networks had high Tc removal capacity, with TcO₄^- trapped within the framework of the sorbent material. Although organoclays did not have the highest capacity for IO₃^- and TcO₄^- removal in batch experiments, they are available commercially in large quantities, are relatively low cost and have low environmental impact, so were investigated in column experiments, demonstrating scale-up and removal of IO₃^- and TcO₄^- via sorption, and reductive immobilization with iron- and sulfur-based species.

Keywords: Bismuth-based materials; Iodate; Iron oxides; Layered double hydroxides; Metal organic frameworks; Pertechnetate.