Equilibrium indium isotope fractionation in chloride-rich aqueous solutions using first-principles calculations

Aqueous indium (In) speciation and isotope fractionation factors are important to understand the origin and transport of indium during magmatic-hydrothermal and atmosphere-hydrosphere processes. Here, we investigate indium speciation in chloride-rich aqueous solutions and explore their reduced partition function ratios (103ln/l) using first-principles calculations. The simulations with initial configurations of InCl(H2O)52+, InCl2(H2O)4+, InCl3(H2O)3, InCl4(H2O)2- and InCl5(H2O)2- were performed at 400 and 600 K, respectively. The results show that InCl(H2O)52+, InCl2(H2O)4+, InCl3(H2O)2.4, InCl4 - and InCl52- are the stable In3+ species at 400 K, and their 103ln/l decreases from 1.00 %o, 0.89 %o, 0.83 %o, 0.75 %o to 0.66 %o. At 600 K, the stable In3+ species are composed of InCl(H2O)52+, InCl2(H2O)3.7+ , InCl3(H2O)1.7, InCl4- and InCl52- with their 103ln/l decreasing from 0.46 %o, 0.42 %o, 0.39 %o, 0.35 %o to 0.31 %o. These results indicate that 103ln/l is negatively correlated with the coordination number and bond length of chlorine ligands in aqueous In3+ species. Meanwhile, water molecules are gradually removed from the first hydration shell gradually with increasing temperature, such as InCl2(H2O)4+ and InCl4(H2O)2- to InCl2(H2O)3.7+ and InCl4-, indicating that stable In3+ species changes with temperature. Hence, estimating the 103ln/l of aqueous liquids at different temperatures should consider the potential structure transition of cation species. The calculations of equilibrium indium isotope fractionation factors provide important insights into the mechanism of isotope fractionation in aqueous liquids, and show the potential application of indium isotopes in the geological and supergene processes of indium.