Chemical Affinity Quantum Sieving with Fluorine for Isotopically Discriminative Removal of Tritium from Radioactive Wastewater

Abstract

The separation of tritium (3H) from radioactive wastewater presents a new challenge for the nuclear and fusion industries. The most promising methods for hydrogen isotope separation are based on isotopic deviations of the zero point energy in chemical bonding that incorporate hydrogen isotopes. This isotope effect can be translated into different formation enthalpies between proton and tritium at isotopically discriminative chemical active sites in solid materials, which is termed as Chemical Affinity Quantum Sieving (CAQS). This research provides a theoretical and experimental demonstration of the chemical affinity quantum sieving effect of fluorine for tritium separation. For the theoretical approach, tritium to protium isotopic reduced partition function (IRPF; s/s’f (T/H)) was calculated based on literature data. This calculation indicated that fluorine (IRPFfluorine = 28.50) is expected to form the highest isotopically discriminative chemical bonding with hydrogen isotopes. Therefore, fluorine was experimentally tested as the best potential candidate for the composition of surface active sites, which could be used for the chemical isotope separation of tritium. For the experimental demonstration, fluorinated MCM‒41 was prepared by thermal treatment with different amounts (n = 0.2 g, 0.5 g, and 1.0 g) of solid NH4F in 60 mL of isopropanol at 120 °C for 24 h (the final product being termed as (n) F‒MCM‒41_NC). We found that fluorinated MCM‒41 shows a significantly enhanced tritium isotope separation factor (α) compared to pristine MCM‒41, indicating the formation of isotopically discriminative chemical active sites by fluorination (αpristine, 6 °C = 1.20, α0.2F, 6 °C = 1.56, α0.5F, 6 °C = 3.29, α1.0F, 6 °C = 1.59). 0.5 F‒MCM‒41_NC synthesized with 0.5 g of NH4F showed an optimized isotope separation factor (α0.5F, 6 °C = 3.29), and demonstrated an excellent compromise between the amount of isotopically discriminative chemical active sites (analyzed using X‒ray diffraction (XRD), Fourier transform infrared (FT‒IR), and solid state 19F MAS NMR) and its stability on the MCM‒41 pore channel (solid state 19F MAS NMR) with the integrity of MCM‒41 frameworks (determined based on BET surface area and TEM image results). The achieved isotope separation factor (α0.5F, 6°C = 3.29) was even higher than that of the chemical isotope exchange reaction of the Girdler sulfide process (α = 2.33) and was comparable to that of the combined electrolysis catalytic exchange (CECE) process (α = 3.7). Experimental and analytical characterization results consistently supported “chemical affinity quantum sieving by the hydrogen isotope exchange reaction at F····OH hydrogen bonding site” as the dominant tritium separation mechanism. The overall upfield shifting of solid state 19F NMR signals indicated the formation of hydrogen bond interactions of F····OH between fluorine species (F‒, [(OH)xSiF6‒x]2‒, SiO3/2F, and SiF4) and the nearby hydroxyl groups generated from sorption reaction. In addition, the FT-IR and 19F NMR signals showed that the counter cation of SiF62‒ was exchanged from NH4+ to oxonium ion (H3O+), which led to the formation of H2SiF6·4H2O during sorption reaction. The formation of H2SiF6·4H2O structure displayed a good positive correlation with the tritium isotope separation factors of (n) F‒MCM‒41_NCs. Moreover, the molecular structure of this newly formed hydrate crystal (H2SiF6·4H2O) was dominated by extensive F····OH hydrogen bonding interaction. Therefore, we suggest that the tritium separation mechanism of (n) F‒MCM‒41_NC is associated with the chemical affinity quantum sieving effect by the hydrogen isotope exchange reaction at F····OH hydrogen bonding site. This research sheds light on the application of FLUORINE as a solid state chemical active site for isotope separation of tritium. We have suggested a mechanistic insight for the future design of novel materials for the selective removal of tritium from radioactive wastewater. In addition, it is expected that this research can be useful for establishing the basis of a new tritium separation process utilizing the solid‒liquid interfacial hydrogen isotope exchange reaction.