Contrasting Zn isotope fractionation effect during adsorption on micro- and nano-goethite

Abstract

Goethite is essential in controlling Zn mobility and behavior in surface environments and can strongly influence Zn isotope fractionation through adsorption processes. However, the influence of particle size of goethite on stable isotopic fractionation remains poorly understood. This research investigates the fractionation of Zn stable isotopes during adsorption onto nano- and micro-goethite, focusing on factors such as reaction duration, Zn concentrations, and pH. The findings reveal that equilibrium in Zn isotope fractionation is achieved after roughly 48 h at pH 6 in both nano- and micro-goethite systems. For nano-goethite, the isotopic fractionation between adsorbed and aqueous Zn (Δ⁶⁶Zn_{adsorbed–aqueous}) remains around −0.1 ± 0.04 ‰, regardless of pH and initial Zn concentrations. During Zn adsorption on micro-goethite, a significantly more negative fractionation of Δ⁶⁶Zn_{adsorbed–aqueous} =  − 0.40 ± 0.04 ‰ is observed. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows that Zn binds to the surface of nano-goethite as a combination of inner-sphere octahedral and tetrahedral surface complexes, with average ZnO bond lengths ranging from 2.00 to 2.02 Å. In contrast, on micro-goethite, Zn predominantly exists as a distorted octahedral structure, exhibiting an average ZnO interatomic distance of 2.09 Å. Our findings suggest that the size-dependent Zn isotope fractionation in goethite is driven by the structural differences in the Zn surface complexes formed on nano- and micro-goethite. These results provide valuable insights into Zn isotope signatures in natural iron-rich soils, supporting the use of Zn isotopes as tracers for understanding the fate of Zn influenced by iron minerals in soil environments.