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

Radioactive iodine-129 (I) and technetium-99 (Tc) 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 I and Tc to reduce environmental mobility. Materials were evaluated with metrics including capacity for IO and TcO uptake, selectivity and long-term immobilization potential. Batch testing was used to determine IO and TcO sorption under aerobic conditions for each material in synthetic groundwater at different solution to solid ratios. Material association with IO and TcO 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 IO. The commercial Purolite A530E anion-exchange resin outperformed all materials in its sorption capacity for TcO. Tin-based materials had high capacity for TcO, but immobilized TcO via reductive precipitation. Bismuth-based materials had high capacity for TcO, though slightly lower than the tin-based materials, but did not immobilize TcO 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 TcO trapped within the framework of the sorbent material. Although organoclays did not have the highest capacity for IO and TcO 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 IO and TcO via sorption, and reductive immobilization with iron- and sulfur-based species.