Abstract: Zinc (Zn) isotope fractionation, resulting from multiple physicochemical processes under Earth’s surface conditions, confounds the source identification of pollutants. The determination of Zn isotope fractionation factors between typical Zncontaining minerals and aqueous solutions in contaminated soils is crucial to trace Zn sources using isotopic tools. In this study, we used density functional theory based first principles calculations to investigate the most stable geometries of Zn-containing species, including hydrated Zn²⁺ complexes, Zn in hydroxy-interlayered minerals (Zn-HIM), Zn-rich phyllosilicates (Talc), Zn-layered double hydroxide (Zn-LDH), and surface adsorbed Zn²⁺. Based on these optimized configurations, we calculated the equilibrium isotope fractionation factors between the aforementioned structures and Zn2+ in aqueous phases. Our results showed that adsorbed Ⅵ Zn²⁺ is slightly enriched in 64Zn (Δ66/64Zn=-0.29‰~-0.20‰ ), while adsorbed Ⅳ Zn²⁺ is enriched in ⁶⁶Zn (Δ^66/64Zn =0.45 ‰~0.73‰ ). Secondary mineral phases are evidently enriched in ⁶⁶Zn (Δ^66/64Zn=0.51‰~1.11‰ ), if Zn transferred to stable crystalline precipitates. Using obtained fractionation factors, we successfully simulated the evolution trends of Zn isotope composition under the influence of single pollution sources (electroplating waste, sphalerite ore, emissions and metallurgical sludge) in equilibrium and Rayleigh fractionation modes. By comparing with available isotope data, these trends facilitate to find out the main source of Zn in contaminated soils.

Key words: Zn migration and transformation, density functional theory, first principles calculation, secondary minerals
precipitation, surface absorption, pollution source identification