Speciation of Rare Earth Elements (REEs) in hydrothermal systems

The Rare Earth Elements (REEs) are an important group of elements both geologically as well as economically. REEs find important applications in the fields of green energy, electric vehicles and electronics. They are also important tracers for geological processes under hydrothermal /high grade metamorphic conditions. The ability of fluids to mobilise the REEs depend on the chemical composition and the presence of suitable ligands like chloride, fluoride. Ab initio molecular dynamics simulations (AIMD) have been used to predict stability constants of various REE complexes and REE speciation under hydrothermal conditions (Stefanski and Jahn, 2020; Guan et al, 2020). However, AIMD simulations often suffer from significant finite time and size effects. Classical potentials on the other hand solve the problems of AIMD to some extent but they often oversimplify the description of atomic interactions. Several interesting phenomena like formation and breaking of bonds are not captured by classical potentials. Hence a good compromise would be to use ReaxFF (Duin et al. 2001). Another potential drawback of simple pairwise additive classical potentials is their implicit description of multipolar effects which play a significant role in solvation of ions in aqueous solvents. These multi-body effects become essential when dealing with multivalent ions, in concentrated solutions and in inter-facial environments (Tazi et al, 2012). Here, we discuss results from preliminary simulations with classical potentials (Migliorati et al. 2017). We demonstrate the finite size and finite time effects associated with AIMD and how classical potentials can be potentially used to overcome them.