Mineralogy and fluid chemistry controls on lithium isotope fractionation during clay adsorption

Our current understanding of controls on δ⁷Li variability and fractionation mechanisms is limited, complicating the interpretation of chemical weathering. The role of clay adsorption in Li isotope fractionation during chemical weathering has been confirmed. However, clay assemblage and fluid chemistry are not simple and often variable in weathering settings, potentially modulating Li isotope fractionation on Earth's surface. Here, this research investigated the patterns and processes of Li isotope fractionation during adsorption on kaolinite and smectite with fluid chemistry of 0.001 M NaCl, 0.5 M NaCl, and 0.001 M Na₂HPO₄. Specifically, the time-dependent experiments with the reaction period up to 15 days revealed that the steady state can be achieved within one day under neutral conditions. The concentration-dependent (initial Li concentration of 2 to 1000 μM) experiments confirmed the accumulation of Li⁺ in smectite interlayers and adsorption of Li⁺ only at the external surfaces of kaolinite. Using 0.5 M NaCl solution and the desorption experiments, we hypothesize that outer-sphere Li may exist in the interlayer sites, which can be replaced by excess Na⁺. In comparison, inner-sphere Li⁺ (unexchangeable) potentially dominates at the edge surface of clays. The presence of Na₂HPO₄ increases the binding capacity for Li⁺ adsorption, in particular for kaolinite. In all cases, ⁶Li is enriched on clay surfaces and interlayer spaces, consistent with field observations. Fluid chemistry may affect the degree of clay Li adsorption but exerted negligible impacts on isotope fractionation. For kaolinite, a wide variation (up to 30 ‰) in isotopic fractionation between adsorbed and aqueous Li (Δ⁷Li_aq-ad) exists, conforming to a kinetic fractionation mechanism with a constant fractionation factor α_ad-aq of ~0.992. By contrast, the isotopic fractionation between Li adsorbed on smectite and Li⁺ left in solutions keeps constant (Δ⁷Li_aq-ad of ~5 ‰), likely following an equilibrium isotope fractionation law with an α_ad-aq of ~0.995.