Si stable isotope fractionation during adsorption and the competition between kinetic and equilibrium isotope fractionation: Implications for weathering systems
Résumé
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 10 3 lnα adsorbed/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
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