Hybrid Sorbents for ¹²⁹I Capture from Contaminated Groundwater

Abstract

Radioiodine (129I) poses a risk to the environment due to its long half-life, toxicity, and mobility. It is found at the U.S. Department of Energy Hanford Site due to legacy releases of nuclear wastes to the subsurface where 129I is predominantly present as iodate (IO3 –). 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 high-performing sorbents for 129I remediation with the capacity to reduce 129I concentrations to or below the US Environmental Protection Agency (EPA) drinking water standard and procedures to deploy them in an ex-situ pump and treat (P&T) system are introduced. This includes implementation of hybridized polyacrylonitrile (PAN) beads for ex-situ remediation of IO3 –-contaminated groundwater for the first time. Iron (Fe) oxyhydroxide and bismuth (Bi) oxyhydroxide sorbents were deployed on silica substrates or encapsulated in porous PAN beads. In addition, Fe–, cerium (Ce)–, and Bi–oxyhydroxides were encapsulated with anion-exchange resins. The PAN–bismuth oxyhydroxide and PAN–ferrihydrite composites along with Fe- and Ce-based hybrid anion-exchange resins performed well in batch sorption experiments with distribution coefficients for IO3 – of >1000 mL/g and rapid removal kinetics. Of the tested materials, the Ce-based hybrid anion-exchange resin was the most efficient for 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 iodine loading capacity. These results highlight the potential for IO3 – remediation by hybrid sorbents and represent a benchmark for the implementation of commercially available materials to meet EPA standards for cleanup of 129I in a large-scale P&T system.

Capture of aqueous radioiodine species by metallated adsorbents from wastestreams of the nuclear power industry: a review

“…It should be noted that the data presented in Table 2 should not be considered the sole determining factor in terms of the desired favourable interactions with aqueous iodine. For example, Fe oxyhydroxides have recently been identified as having potential in this remit and cerium has been used historically for iodate immobilisation [35].…”

“…Further recent research has demonstrated a different loading technique, in which the commercial resins were treated with an acidic solution of metal ions, followed by NaOH. This caused precipitation of metal oxides upon the adsorbent surface, creating a true hybrid material [35]. Some less common metals were trialled, including Bi, Ce and Fe, with a CeO 2 -loaded resin proving the most efficient material.…”

“…The first parameter ranges from 0.2-200 g adsorbent per L of water treated, while the second has been studied from 0.5-12,700 mg L −1 . This is because the objective behind some work is to demonstrate that iodine concentration can be reduced to below detectable or acceptable levels, with a sufficient adsorbent mass [35], whereas other researchers place more value in the maximal adsorption capacity for the material [23], while both approaches yield valuable data, it does make direct comparison of adsorbents problematic.…”

“…In this case, adsorbents with a workable particle size and faster kinetics would be desirable. However, for management of a finite amount of low-level legacy aqueous waste in temporary containment, where water-treatment rate is not critical (as found at the US Department of Energy Hanford site), a batch system could offer almost complete removal of iodine, with high efficiency [35].…”

“…In contrast, basic conditions result in iodide and iodate as the two dominant species. A small number of studies, in recognition of this, have examined iodate removal or compared the uptake of both species [20,25,35]. Whilst it is clear that the metallated materials show ability to capture and retain both anions, the small amount of data make it very difficult to draw conclusions as to which adsorbent types might be more appropriate to remediate one species over the other.…”