The Theoretical Calculation of the Cu Isotope Fractionation Effect in Solution/Hydrothermal Solution Systems

Copper (Cu) is an important transition metal, and its isotopes have important applications in geology, environmental science, soil science, and other fields. Cu isotope fractionation can occur in many natural processes. However, the mechanism of Cu isotope fractionation in solution/hydrothermal solution systems is not very clear. In this study, the fractionation effects of complexes of Cu(I) and Cu(II) in solution/hydrothermal solution systems were systematically studied by means of an ab initio method based on first principles. In the simulation of an aqueous solution system, the theoretical treatment method used is the “water-droplet” method. The results show that the heavy Cu isotope (⁶⁵Cu) enrichment capacity of the Cu-bearing complex solutions is greatly affected by the ligand types both for Cu(I) and Cu(II). For Cu(I) complex solutions, the heavy Cu isotope enrichment sequence is [Cu(HS)₂]⁻·(H₂O)₄₂ > [Cu(HS)(H₂O)]·(H₂O)₄₂ ≈ [Cu(HS)(H₂S)]·(H₂O)₄₂ > [CuCl]·(H₂O)₄₂ > [CuCl₂]⁻·(H₂O)₄₂ > [CuCl₃]²⁻·(H₂O)₄₂. For the aqueous solutions of Cu(II) with an inorganic ligand (such as H₂O, OH⁻, NO₃⁻, SO₄²⁻ and CN⁻), the order of heavy Cu isotope enrichment is as follows: [Cu(H₂O)₆]²⁺·(H₂O)₄₂ > [Cu(NO₃)₂]·(H₂O)₄₂ > [Cu(OH)₂]·(H₂O)₄₂ > [CuSO₄(H₂O)₃]·(H₂O)₄₂ > [CuNO₃(H₂O)₄]⁺·(H₂O)₄₂ > [CuCN]⁺·(H₂O)₄₂. For the Cu(II) complex solutions with a halogen as ligands, the change order of 1000lnβ is [CuCl]⁺·(H₂O)₄₂ > [CuCl₂]·(H₂O)₄₂ > [CuBr₂]·(H₂O)₄₂ > [CuCl₃]⁻·(H₂O)₄₂. The sequence of 1000lnβ for Cu(II) organic complex aqueous solutions is [Cu(HOC₆H₄COO)]⁺·(H₂O)₄₂ > [Cu(CH₃CH₂COO)]⁺·(H₂O)₄₂ > [Cu(COOHCOO)]⁺·(H₂O)₄₂. The calculation also found that for Cu(I) complex aqueous solutions, the difference in Cu isotope fractionation parameters (1000lnβ) between [CuCl₂]⁻·(H₂O)₄₂ and [Cu(HS)₂]⁻·(H₂O)₄₂ is relatively large. At 100 °C, the 1000lnβ of the two species are 1.14 and 1.55 (‰), respectively. The difference between the two could be reached up to 0.41 (‰). The Cu isotope fractionation parameter obtained with the “water droplet” method is also very different from the results of previous studies, which indicate that the Cu isotope fractionation behavior of the two is similar. At the same time, the exciting discovery is that the enrichment capacity of heavy Cu isotopes is significantly different between Cu(I) complex aqueous solutions and Cu(II) complex aqueous solutions. At 100 °C, the 1000lnβ of 6 Cu(I) complex aqueous solutions and 13 Cu(II) complex aqueous solutions ranged from 0.90 to 1.55 and 2.24 to 3.25(‰), respectively. It also shows that the REDOX reaction has a significant effect on the Cu isotope fractionation, especially in ore-forming fluids. Therefore, the ligand type is a factor that cannot be ignored when considering the mechanism of Cu isotope fractionation in solution/hydrothermal solution systems. Whether the solvation effect of an aqueous solution is considered or not has a great influence on the numerical values of the final Cu isotope fractionation factors. Hence, the solvation effect of an aqueous solution is an essential determinant in the theoretical calculation of the Cu isotope fractionation factors for Cu-bearing complex solutions.