Biotite surface chemistry as a function of aqueous fluid composition

The chemical composition and charge of the biotite near-surface, in contact with NaCl bearing aqueous solutions at 25°C from pH 1 to 12, have been derived via zeta potential measurements and potentiometric titrations performed for 20 and 60min in batch reactors. Zeta potential measurements yielded an isoelectric point of pH 3.0 (±0.2) and batch potentiometric titrations yielded a pH of immersion of 9.66 (S.D. 0.24). From batch potentiometric titrations we determined both the proton consumption and the metal release from the biotite surface as a function of pH. Potassium removal from the near-surface of biotite is only slightly dependent on pH with a minimum of ~6atomsnm removed at the immersion pH, corresponding to an average depletion depth of ~1.5nm. In contrast, the release of Mg, Al and Fe is strongly pH-dependent as those metals are preferentially removed from the biotite surface at pH less than 9 (Mg) and 4 (Al, Fe). The average depletion depth of Mg, Al, and Fe increases with decreasing pH, reaching on average ~2nm at pH ~1. The removal of K, Mg, Al, and Fe is not charge conservative, resulting in a relative negative charge in the biotite near-surface. Taken together, our results indicate that the composition of the biotite surface varies dramatically as a function of pH. At basic conditions, the biotite near-surface is K depleted and likely hydrogen enriched. At near-neutral conditions, the biotite near-surface is comprised of only the Si and Al tetrahedral, and the Fe(II) octahedral framework, following the removal of both alkali metals and Mg. Finally, at acidic conditions, the biotite near-surface is comprised exclusively of a remnant Si, O and H framework. The results of these experiments give an indication of the composition and charge of the biotite surface in the natural environment, following contact with water, for example in the vadose zone, and can help us understand weathering reactions in these systems.