Sn isotope fractionation during volatilization of Sn(IV) chloride: Laboratory experiments and quantum mechanical calculations

Volatilization is an important pathway of element transport in nature, and this process may be associated with stable isotope fractionation, which could be used to understand the elemental volatilization mechanisms. In this study, we report that evaporation of Sn(IV) chloride solution under experimental conditions (96 degrees C, 1 atmospheric pressure) results in significant loss of aqueous Sn(IV) and Sn stable isotope fractionation. The delta Sn-122/116 of the residue solution can increase by up to 0.50% (or a 0.33% increase in delta Sn-122/118) after repeated evaporation, indicating preferential partitioning of isotopically light Sn species into the vapor phase during evaporation. The observed Sn loss and associated Sn isotope fractionation during the evaporation experiments can be described using a Rayleigh fractionation function, with a best-fitting isotope fractionation factor of -0.36% in Delta Sn-122/116(vapor-aq) (or -0.24% in Delta Sn-122/118(vapor-aq)). We also performed quantum mechanical calculations to assess the stability of different potential Sn(IV) species in aqueous and vapor phases, and derived the equilibrium Sn isotope fractionation factors between these Sn(IV) species. The calculation results suggest that the dominant gaseous species of Sn (IV), SnCl4, is isotopically heavier than the aqueous Sn(IV) species by 1.24%-0.35% in delta Sn-122/116 (or 0.82%-0.19% in delta Sn-122/118) under equilibrium at 96 degrees C, which is opposite to the experimental results. Such contrast in the direction of Sn isotope fractionation implies that kinetic isotope fractionation, rather than thermodynamic equilibrium isotope fractionation, took place for SnCl4 in the evaporation experiments at 96 degrees C. The observed experimental data can be explained by a kinetic isotope fractionation model involving backward reaction of SnCl4 vaporization at the solution-vapor boundary. This study, in combination with a recent report of positive Delta Sn-122/116(vapor-aq) (or Delta Sn-122/118(vapor-aq)) factor during evaporation of SnCl4 at 150 degrees C (Wang et al., 2019a), suggests that Sn(IV) volatilization mechanisms may be different with and without fluid boiling. Combined laboratory experiments and quantum mechanical calculations on the isotopic effects of Sn(IV) chloride solution evaporation provide important constraints for understanding the rapidly accumulating Sn isotopes data from studies on Sn ore-forming processes, bronze metallurgy and archeology, and volatile elements in planetary processes. (C) 2019 Elsevier Ltd. All rights reserved.