Theoretical Calculation of Equilibrium Cadmium Isotope Fractionation Factors between Cadmium-bearing sulfides and aqueous solutions

Abstract

In this study, the equilibrium isotope fractionation factors between Cd-bearing aqueous solutions and minerals were predicted. The theoretical method used to calculate the Cd isotope fractionation factors is the first-principle quantum chemistry method (Cd: LANL2DZ, other atoms: 6-311 + G(d, P)). Reduced partition function ratios (RPFRs) of Cd-bearing minerals (Greenockite and Sphalerite) were modeled by the method of the volume variable cluster model (VVCM). The theoretical method of “water-droplet method” is used to simulate the solvation effect of different Cd-bearing aqueous solutions. The results show that, in most cases, the Cd-bearing aqueous solutions are enriched in ¹¹⁴Cd relative to Greenockite. The Cd isotope fractionation factors between Cd-bearing aqueous solutions and Greenockite are in the range of 0.433–0.083 (100°C). And the Cd isotope fractionations between different Cd-bearing species are believed to be widespread. Cd isotope fractionation factors between different reservoirs are of great theoretical significance to many geochemical processes such as surficial geochemical process and ore-forming process. These theoretical parameters are studied systematically and carefully in this study.