A Model of Enhanced Silicate Weathering in Soil to Sequester Atmospheric CO2 as Soil Carbonate

Abstract:

One of the most promising ‘geo-engineering’ methods to mitigate climate change is to capture CO₂ from the air and react it with silicate rocks to form carbonates in so-called enhanced weathering, so permanently sequestering the CO₂. Soils are a good source of CO₂ for this purpose because C fixed by plants in photosynthesis and released in root and soil respiration results in the CO₂ pressures in soil air being 10–1000 times that in the bulk atmosphere. However, the necessary conditions for this to work are not well understood.

Current models of mineral weathering over-predict rates of silicate weathering in soils by 10–100 fold under laboratory and field conditions. The current models are mostly based on simple solution systems, in which dissolution is congruent and rates are limited by reaction mechanisms at the mineral surface. Whereas in soils, dissolution is rarely congruent, silicate dissolution products typically being re-precipitated in less-soluble phases; and, since rates of diffusion are typically orders of magnitude slower in soil than in solution, diffusion of dissolution products away from the dissolving surface, or of reactants towards it, is likely to be rate-limiting. This means current models both over-predict rates of dissolution and hence atmospheric CO₂ fixation, and incorrectly predict sensitivities to the important soil, vegetation and environmental variables.

I here present a model of silicate weathering in soil allowing for diffusion as a rate-limiting process. The model assumes soluble silicates, for which surface dissolution processes are rate-limiting, dissolve and the dissolution products are re-precipitated as carbonates, for which diffusion processes are rate-limiting. The carbonates occlude the remaining residual silicates. The continuing dissolution therefore depends on the diffusion of dissolution products away from or reactants toward the re-precipitation zone. The contraction of the precipitation zone over time as dissolution proceeds, and the gradual overlap between the zones of influence of neighbouring silicate particles, are allowed for. The model satisfactorily accounts for published rates of dissolution of an olivine in a calcareous soil, for which the standard model based on simple solution kinetics, over-predicts dissolution by two orders of magnitude.