Equilibrium boron isotope fractionation during kaolinite adsorption and applications to chemical weathering processes

Abstract

The chemical weathering of continental silicates affects the global climate and the geochemical cycle of elements, and because of their significant fractionation during chemical weathering, boron isotopes have great potential to trace chemical weathering processes. However, our understanding of boron isotope fractionation in the weathering environment is limited. In this study, the equilibrium boron isotope fractionation between the kaolinite (010) surface and aqueous fluids is investigated using quantum mechanics calculations (density functional theory, DFT). On the basis of the computation of binding energy and interface-structure optimization, a three-step reaction mechanism of boron adsorption on kaolinite is proposed: i) the free hydronium ions bond to the surface hydroxyl groups of kaolinite to form water molecules at mineral surface; ii) both B(OH)₃ and B(OH)₄⁻ replace the water molecule on the mineral surface via the coordination hydroxyl group; iii) the free hydroxyl groups capture the hydrogen ions from bridging oxygen to form free water molecules. We first verified the influence of the B-O bond length and the O-B-O angle distortions on boron isotope fractionation and observed negative correlations between the magnitude of modelled boron isotope fractionation and both the average B-O bond length and the degree of structure distortion. Comparison between theoretical and experimental results indicates that the BO₄ structure dominates the boron isotope fractionation at the kaolinite (010) surface. This, in turn, allows for a more detailed understanding of boron isotope fractionation in the surface weathering environment at different T, pH and salinity conditions.