Coupling of Li-Fe: Li Isotope Fractionation during Sorption onto Fe-Oxides

Abstract

Chemical weathering processes play a key role in regulating the global climate over geological time scales. Lithium (Li) isotope compositions have proven to be a robust proxy for tracing weathering processes that produce secondary minerals, such as clays and oxides, with a focus often placed on Li adsorption to, or incorporation into, clay minerals. In addition, the interaction between Li and Fe-oxides has long been assumed and discussed based on field observations, but experimental constraints on this process are lacking. Here, we investigated the geochemical behavior of Li during its sorption onto individual Fe-oxides, including goethite, hematite, wüstite, and magnetite. With a point of zero charge at ∼7.7, poorly crystallized goethite nanoparticles take up ∼20% of dissolved Li over a pH range from ∼4 to ∼10, rising to ∼90% at pH ∼12. In contrast, the sorption of dissolved Li is insignificant for well-crystallized Fe-oxides (hematite, wüstite, magnetite, and goethite). This Li uptake by poorly crystallized goethite is likely attributed to dissolution and reprecipitation reactions at poorly crystalline goethite surfaces. The goethite particles preferentially take up light 6Li isotopes, resulting in an isotope fractionation of Δ7Lioxide-fluid ∼ −16.7 to −20.1‰. Overall, our study provides valuable data to better understand the processes occurring in highly weathered soil and sediment profiles that are rich in Fe-oxides, such as laterites. This research also emphasizes the significance of chemistry at mineral surfaces during mineral-water interactions and illuminates the mechanisms of large-scale Li extraction for future applications.