PNNL

Abstract

Radioiodine (129I) poses a risk to groundwater due to its long half-life, toxicity, and environmental mobility. 129I is present as iodate (IO3-) in groundwater at the Hanford Site, WA due to release of nuclear weapons production wastes to the subsurface. To date, a cost-effective and scalable cleanup technology for 129I has not been identified, with hydraulic containment implemented as the remedial approach. Here, novel active sorbents and procedures to deploy them for 129I remediation are introduced. High performing sorbents with capacity to reduce 129I concentrations from average groundwater concentrations to the US Environmental Protection Agency (EPA) drinking water standard were hybridized into three different forms suitable for deployment in an ex-situ pump and treat (P&T) system. This included implementation of hybridized polyacrylonitrile (PAN) beads for IO3- remediation in groundwater for the first time. Iron oxy(hydroxide) and bismuth oxy(hydroxide) powders were deployed on silica substrates or encapsulated in porous PAN beads. In addition, iron-, cerium and bismuth-(oxy)hydroxides were hybridized with anion exchange resins. Active sorbent crystallinity and distribution within deployable forms were assessed by X-ray diffraction and scanning electron microscopy respectively. The mechanism of iodine capture was predominantly sorption, as confirmed by identification of IO3- on the solid phases by iodine K-edge X-ray absorption spectroscopy. Hybrid sorbents were subjected to batch sorption, kinetics and flow-through column experiments to evaluate performance for IO3- capture. The PAN-bismuth (oxy)hydroxide and PAN-ferrihydrite composites, along with iron- and cerium-based hybrid anion exchange resins performed well in batch sorption experiments. with distribution coefficients for iodine of > 1000 mL/g and rapid removal kinetics. Of the tested materials, Cerium-based hybrid anion exchange resin showed the most removal of IO3- from Hanford groundwater in a column system, with 50% breakthrough occurring at 324 pore volumes. The functional amine groups on the parent resin, and amount of active sorbent in the resin, can be customized to improve the observed low iodine loading capacity. These results highlight the potential for IO3- remediation by hybrid sorbents and represent a benchmark for implementation of commercially available materials to meeting EPA standards for cleanup of 129I in a large-scale P&T system.