Zn Isotope Fractionation during Sorption onto Kaolinite

In this study, we quantify zinc isotope fractionation during its sorption onto kaolinite, by performing experiments under various pH, ionic strength, and total Zn concentrations. A systematic enrichment in heavy Zn isotopes on the surface of kaolinite was measured, with Δ⁶⁶Zn_{adsorbed‑solution} ranging from 0.11‰ at low pH and low ionic strength to 0.49‰ at high pH and high ionic strength. Both the measured Zn concentration and its isotopic ratio are correctly described using a thermodynamic sorption model that considers two binding sites: external basal surfaces and edge sites. Based on this modeling approach, two distinct Zn isotopic fractionation factors were calculated: Δ⁶⁶Zn_{adsorbed‑solution} = 0.18 ± 0.06‰ for ion exchange onto basal sites, and Δ⁶⁶Zn_{adsorbed‑solution} = 0.49 ± 0.06‰ for specific complexation onto edge sites. These two distinct factors indicate that Zn isotope fractionation is dominantly controlled by the chemical composition of the solution (pH, ionic strength).