Zinc Stable Isotope Fractionation Mechanisms during Adsorption on and Substitution in Iron (Hydr)oxides

The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (Δ^66/64Zn_{solid-solution} = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with Δ^66/64Zn_{substituted-stock solution} of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.

Phosphorus Release and Speciation in Manganese(IV) Oxide and Zeolite-Amended Flooded Soils

Phosphorus (P) losses from flooded soils and subsequent transport to waterways contribute to eutrophication of surface waters. This study evaluated the effectiveness of MnO₂ and a zeolite Y amendment in reducing P release from flooded soils and explored the underlying mechanisms controlling P release. Unamended and amended (MnO₂ or zeolite, surface-amended at 5 Mg ha^-1) soil monoliths from four clayey-alkaline soils were flooded at 22 ± 2 °C for 56 days. Soil redox potential and dissolved reactive P (DRP), pH, and concentrations of major cations and anions in porewater and floodwater were analyzed periodically. Soil P speciation was simulated using Visual MINTEQ at 1, 28, and 56 days after flooding (DAF) and P K-edge X-ray absorption near-edge structure spectroscopy and sequential fractionation at 56 DAF. Porewater DRP increased with DAF and correlated negatively with pe+pH and positively with dissolved Fe. Reductive dissolution of Fe-associated P was the dominant mechanism of flooding-induced P release. The MnO₂ amendment reduced porewater DRP by 30%-50% by favoring calcium phosphates (Ca-P) precipitation and delaying the reductive dissolution reactions. In three soils, the zeolite amendment at some DAF increased porewater and/or floodwater DRP through dissolution of Ca-P and thus was not effective in reducing P release from flooded soils.