Mechanism of boric acid extraction by trioctylamine and tartaric acid

The separation of boron isotopes poses significant challenges due to their similar chemical properties. Previous studies have demonstrated that boron isotope separation can be achieved by solvent extraction of boric acid (H3BO3) using trioctylamine (TOA) and tartaric acid (DL-TA), resulting in a remarkable separation factor of up to 1.04 through chemical exchange. However, the coordination mechanism among these three compounds remains elusive, impeding our understanding of the isotope exchange process. In this study, the species in the aqueous and organic phases were investigated using extraction experiments (maximum loading method), alongside various characterization techniques (FT-IR, B-11 NMR, and high-resolution ESI-MS), followed by DFT computations. The results showed that H3BO3 and DL-TA formed anionic complexes in the aqueous phase with stoichiometric ratios of 1:1 and 1:2, respectively. However, the stoichiometric ratios became more intricate in the organic phase, encompassing 1:1, 1:2, 2:2, 2:3, and 3:3. The average stoichiometric ratio of H3BO3 to DL-TA calculated within the range of 1:1 to 1:1.5. Protonated TOA could form ion pairs with these anionic complexes, while unprotonated TOA might form hydrogen bonds with the carboxyl groups in DL-TA, contributing to the complexity of the stoichiometric ratios between H3BO3 and TOA. The DFT calculations indicated that all complexes containing B-(4)-O bonds, including 1:1, 1:2, 2:2, 2:3, and 3:3, effectively facilitated the exchange of isotopes with H3BO3. These findings pave the way to process optimization of boron isotope separation by solvent extraction of boric acid.