The effect of pH, grain size, and organic ligands on biotite weathering rates

Biotite dissolution rates were determined at 25°C, at pH 2-6, and as a function of mineral composition, grain size, and aqueous organic ligand concentration. Rates were measured using both open- and closed-system reactors in fluids of constant ionic strength. Element release was non-stoichiometric and followed the general trend of Fe, Mg>Al>Si. Biotite surface area normalised dissolution rates (r_i) in the acidic range, generated from Si release, are consistent with the empirical rate law:ri=kH,iaH+xiwhere k_H,i refers to an apparent rate constant, aH+ designates the activity of protons, and x_i stands for a reaction order with respect to protons. Rate constants range from 2.15×10^-10 to 30.6×10^-10(moles_biotitem^-2s^-1) with reaction orders ranging from 0.31 to 0.58. At near-neutral pH in the closed-system experiments, the release of Al was stoichiometric compared to Si, but Fe was preferentially retained in the solid phase, possibly as a secondary phase. Biotite dissolution was highly spatially anisotropic with its edges being ~120times more reactive than its basal planes. Low organic ligand concentrations slightly enhanced biotite dissolution rates. These measured rates illuminate mineral-fluid-organism chemical interactions, which occur in the natural environment, and how organic exudates enhance nutrient mobilisation for microorganism acquisition.