Si stable isotope fractionation during adsorption and the competition between kinetic and equilibrium isotope fractionation: Implications for weathering systems

The adsorption of Si onto amorphous Al-hydroxides is the cause for the light Si isotope signature that secondary crystalline clay minerals in weathering systems carry. We propose this hypothesis from a series of adsorption experiments in which the light isotopes are being favored during Si adsorption onto crystalline gibbsite and in which the associated fractionation factor depends on the solution's initial Si concentration.

Three adsorption experiments were carried out at pH 7 with different initial Si concentrations of 0.36, 0.71 and 1.42 mmol/l Si start concentrations. As Al-hydroxide adsorbent, 30 g/l crystalline gibbsite were used to provide equal surface area in all experiments. Adsorption rates are higher with higher initial Si concentration. At the same time, calculated apparent isotope fractionation factors 103 ln αadsorbed/solution decrease from − 1.8 to − 3‰ with increasing initial Si concentration. As care was taken to avoid isotope fractionation during transport of dissolved Si to the gibbsite surface, the mass dependence of the activation energy barrier at the interface is causing the kinetic isotope fractionation. Within the mass balance framework of DePaolo (2011) the shift in Si isotope fractionation with initial Si concentration is interpreted to be induced by different kinetic isotope fractionation factors associated with the forward reaction. Only after ca. two months do the isotope ratios begin to adjust to an equilibrium isotope fractionation factor that is close to 0‰. With such slow re-equilibration Si adsorption differs fundamentally from transition metals that re-equilibrate isotopically within hours after adsorption onto Fe and Mn oxide surfaces.

These observations may provide an explanation for the light Si isotope signature clay minerals formed during weathering carry: the light Si isotope composition is being inherited early on during Si adsorption onto amorphous Al-hydroxides and is potentially carried over during all further stages of transformation.