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

Abstract

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:where k H,i refers to an apparent rate constant, a H þ designates the activity of protons, and x i stands for a reaction order with respect to protons. Rate constants range from 2.15 Â 10 À10 to 30.6 Â 10 À10 (moles biotite m À2 s À1 ) 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 $120 times 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.

Impact of grain size and rock composition on simulated rock weathering

“…These values are reflective of the kinetic rate of dissolution, so that 1 represents the maximum rate of dissolution and 0 represents an inert state. In our formulation, grain boundaries dissolve 1 order of magnitude more rapidly than the mineral core, which is consistent with experimental work (Lüttge et al, 2013;Emmanuel, 2014;Bray et al, 2015).…”

The efficient long-term inhibition of forsterite dissolution by common soil bacteria and fungi at Earth surface conditions

“…It has nevertheless been suggested that fluids can become supersaturated locally if they are isolated from the bulk fluid leading to mineral replacement reactions (e.g. Putnis 2009;Ruiz-Agudo et al, 2012;Bray et al, 2014Bray et al, , 2015.…”

Ectomycorrhizal Fungi and Mineral Interactions in the Rhizosphere of Scots and Red Pine Seedlings

“…Weathering rates were expressed as element release rates, determined for each treatment in each experiment in mmol/m 2 of mineral surfaces [61,62]. The total amounts of calcium and potassium were calculated in irrigation and drainage waters, biomass, and soil exchange sites; then, the total release was determined by a simple mass balance of output minus input for all pools [30].…”

“…Biotite tends to dissolve and transform easily in any acidic environment [61,[66][67][68], and increased dissolution rates have been demonstrated with the presence of organic acids, especially oxalate [61,69,70]. However, biotite persists in many soils, which indicates that biotite dissolution is incongruent in pH conditions common to soils and element release becomes diffusion-limited due to secondary phases/remnant silicate framework at the mineral surface [61].…”

“…However, biotite persists in many soils, which indicates that biotite dissolution is incongruent in pH conditions common to soils and element release becomes diffusion-limited due to secondary phases/remnant silicate framework at the mineral surface [61]. Some agricultural studies showed that after tillage, the soils released more cations from clays [71], and in forests after disturbance, the cation nutrient release also increased for a short period of time [25,45,72], because these processes likely expose fresh mineral surfaces and enhance the temporary cation release from both primary and secondary minerals.…”