Selective extraction of lithium from seawater desalination concentrates: study of thermodynamic and equilibrium properties using Density Functional Theory (DFT)

Abstract

Lithium, declared critical raw material by the European Union in 2020, is a competitor to hydrogen as alternative to petroleum. Its use is increasing while reserves are declining, boosting new sources, as seawater desalination concentrates. In this work, a computational study of the most promising extractants, B-diketones and organophosphates and combinations thereof, towards lithium in presence of metal ions found in the concentrates, Na+, K+, Mg2+, Ca2+ and Sr2+ was carried out, via molecular simulation using ab initio Density Functional Theory (DFT). The geometries, reaction energies, and thermodynamic parameters have been evaluated. Using the square of the electronic wave function an electrostatic interaction was confirmed as cation-extractant/s bonding. The complexation reaction energies of the systems formed by a cation and a single extractant display negative [delta]E and [delta]G values, pointing towards stable complexes and spontaneous reactions. The synergic effect of extractants was studied by combining the [beta]-diketones with TOPO (1:1) leading to an increase of [delta]E and [delta]G (absolute value). The extraction coefficient, K, follows the order K(K+) > K(Na+) > K(Li+) > K(Sr2+) > K(Ca2+) > K(Mg2+). In consequence, selectivity Li+ towards cations of the group II was higher, S(Li+/Mg2+) > S(Li+/Ca2+) > S(Li+/Sr2+) for the combined mixtures BTA TOPO and FDOD TOPO and lower towards group I cations, S(Li+/Na+) > S(Li+/K+) for DBM TOPO and LIX54 TOPO. The selectivity of Li+ regarding the rest of the cations and the 16 extractants and mixtures of extractants was lower than the selectivity of Li+ with respect to each cation, being the best value for the DBM TOPO and LIX54 TOPO systems. The results obtained are expected to provide a tool on the behaviour of the most promising of extractants towards Li+ in seawater desalination concentrates.