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

Abstract

Iodine-129 poses a significant challenge in the drive towards lowering radionuclide emissions from used nuclear fuel recycling operations. Various techniques are employed for capture of gaseous iodine species, but it is also present, mainly as iodide anions, in problematic residual aqueous wastestreams, which have stimulated research interest in technologies for adsorption and retention of the radioiodine. This removal effort requires specialised adsorbents, which use soft metals to create selectivity in the challenging chemical conditions. A review of the literature, at laboratory scale, reveals a number of organic, inorganic and hybrid adsorbent matrices have been investigated for this purpose. They are functionalised principally by Ag metal, but also Bi, Cu and Pb, using numerous synthetic strategies. The iodide capacity of the adsorbents varies from 13 to 430 mg g−1, with ion-exchange resins and titanates displaying the highest maximum uptakes. Kinetics of adsorption are often slow, requiring several days to reach equilibrium, although some ligated metal ion and metal nanoparticle systems can equilibrate in < 1 h. Ag-loaded materials generally exhibit superior selectivity for iodide verses other common anions, but more consideration is required of how these materials would function successfully in industrial operation; specifically their performance in dynamic column experiments and stability of the bound radioiodine in the conversion to final wasteform and subsequent geological storage. Article highlights Metallated adsorbents for the capture and retention of radioiodine in the nuclear industry are assessed. The strengths and weaknesses of organic, inorganic and hybrid support matrices and loading mechanisms are discussed. Pathways for progression of this technology are proposed. Graphic abstract

Functionality screening to help design effective materials for radioiodine abatement

“…It is important to note that, within the intended iodine containment process, the efficiency of the adsorbent for aqueous iodine removal is not the only criterion. The compatibility of the spent adsorbent with a suitably inert wasteform, to avoid leaching of radioiodine into the environment is also of huge importance ( Riley et al, 2015 ; Robshaw et al, 2021a ) and there are questions to be asked of how the various organic ligands will interact with what will likely be an inorganic wasteform chemistry ( Abdel Rahman and Zaki, 2020 ).…”

“…The uptake capacity data from the first round can be compared to calculated isothermal q max values reported for other adsorbents in the literature field with some confidence. The q max term represents the theoretical saturation capacity of an adsorbent, calculated from a recognised isotherm equation and is widely used in the field of adsorbent development for iodine capture ( Robshaw et al, 2021a ). In first round experiments, the residual iodide concentration in the sample solutions for every material was >>0 mg L −1 .…”

“…A breakdown of the composition of each contact solution is shown in Table 2 . The high and low values for the continuous exploratory variables were determined either by known limitations of the adsorbents (For example, the inorganic matrix forms of a number of these adsorbents are known to dissolve at pH > 12 ( Lambert, 2020 )) or by the ranges reported in the literature ( Robshaw et al, 2021a ). It should be noted that Table 2 is only a representation of the experimental design and we do not claim the values are accurate with respect to the actual measured concentration of each analyte.…”

“…There is however no effective method for the removal of the remaining fraction of iodine in the aqueous phase. In UNF reprocessing, the dissolver raffinate travels through various separation stages, with further iodine releases into the off-gas streams ( Robshaw et al, 2021a ). Therefore, an aqueous iodine-removal step could hypothetically be inserted at many stages of unit operations.…”

“…An effective adsorbent for aqueous radioiodine must be robust enough to (at least temporarily) withstand harsh chemical and radiolytic conditions, selective enough to retain appreciable iodine capacity in the presence of competitor species and be amenable to conversion to a stable final wasteform. A number of materials have been researched in recent years for this remit, which have been recently reviewed ( Robshaw et al, 2021a ). The huge majority incorporate iodine-selective metals into the adsorbent to create the required selectivity, with the most popular metals being Ag, Bi, Cu and Pb ( Kodama, 1999 ; Lefevre et al, 1999 ; Zhang et al, 2011 ; Al-Mamoori et al, 2020 ).…”