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

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