Insights into the mobility of rare earth complexes in groundwater-rock interactions: A geochemical view from modeling of computational chemistry

This study investigates the prediction and stability of novel complexes of rare earth elements (REE) under simulated groundwater and hydrothermal conditions. The electronic structure properties of the REE complexes LnX₃ and Ln₂Y₃, with Ln = La, Lu; X = Cl^-, F^-, (HCO₃)^-, (AlO₂)^-, and Y = (SO₄)^2-, (CO₃)^2- were explored using density functional theory (DFT) including relativistic effects. The nature of the bonding in the complexes was assessed via the theory of atoms in molecules. Evidence shows that relativistic contributions play a significant role in stabilization energy and bonding distance, especially in La-complexes, where bond distances shrink by 0.64-6.82 %. Formation energies were calculated to identify the most stable configurations, being the most stable Ln₂ (SO₄)₃. Molecular electrostatic potential (MEP) was analyzed to predict the acidic or basic behavior of the complexes. Ab initio molecular dynamics (AIMD) simulations were also performed for models with the first solvation shell, and also for models representing massive systems. In large-scale systems at 300 °C, the stability varies by element. For La, the most stable is La₂(CO₃)₃, while for Lu, it is Lu₂(SO₄)₃ ≈ Lu₂(CO₃)₃. It evidences the feasibility of the existence of the REE complexes in aqueous media. These results give insight into the mobility and stability that these complexes may present in water-rock interaction processes of groundwater and environmental systems.