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

Abstract

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 SO42− 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