Density functional theory study with and without COSMO of H2SO4 reactions in an aqueous environment for metal extraction

In a recent study investigating the suitability of solvent extraction (SX) for the separation of Ta and Nb, it was shown that speciation data would be required to help explain the data obtained. As traditional speciation techniques cannot be readily applied for Ta and Nb, it was decided to determine the suitability of molecular modeling for this purpose. During the SX experiments the aqueous phase consisted of sulfuric acid (H₂SO₄), water, and metal species. In this study density functional theory (DFT) modeling was used to calculate the formation energy of five possible reactions of H₂SO₄ and H₂O. Different functional and basis set combinations were compared as well as the effect of infinite dilution by using the conductor-like screening model (COSMO), which simulates infinite dilution of solvents of varying polarity and includes the short-range interactions of the solute particles. The results obtained were used to determine whether it is possible to predict the reactions and mechanism when H₂SO₄ and H₂O interact during SX. According to the results, the deprotonation of H₂SO₄ was endothermic in a 1:1 acid–water ratio, while being both exothermic in the 1:5 and 1:10 acid–water ratio forming HSO₄^? and SO₄^2? respectively. Furthermore, it was seen that the hydration and dehydration of H₂SO₄ in a bulk H₂O solution was a continuous process. From the energy calculations it was determined that although the H₂SO₄●H₂O, HSO₄^?●H₂O, and H₂SO₄●2H₂O species could form, they would most likely react with H₂O molecules to form HSO₄^?, H₃O⁺, and H₂O. © 2018 Wiley Periodicals, Inc.