PORE SCALE VISUALIZATION OF MULTIPHASE REACTIONS IN THE VADOSE ZONE

Earth’s shallow subsurface is a conduit between the atmosphere and hydrosphere wherein reactions instrumental for nutrient availability, water quality, and carbon sequestration occur¹. Reasonable predictions of the progress of such reactions require reactive transport models that capture the governing mechanisms. Here, we use novel microfluidic techniques to elucidate the impact of evaporative processes on CO₂ dissolution and fixation in the vadose zone at the pore scale and identify a number of previously unheralded processes that exert considerable influence on reaction rates. The reactivity of gaseous CO₂ with brucite [Mg(OH)₂] slurries, a mineral of interest for carbon sequestration², was investigated in a variably water-filled microfluidic pore network (i.e., micromodel). The results of this study confirm that micromodel experiments are a valuable technique to elucidate dynamics of gas-water-mineral reactions in the vadose zone. We show that the dynamic evolution of the gas-water interface, water content, and mineral to water ratio during evaporation may significantly alter reaction rates. Observed mobility and migration of brucite particles implies that reactivity may be enhanced during evaporation, counter to an inhibitory effect due to water loss. An increased proportion of particles at the gas-water interface may inhibit gas dissolution, yet increased porosity in dry pores may enhance relative permeability with respect to the gas phase. We conclude that water limited reaction and particle movement may be key factors governing overall reactivity in low water environments. The omission of these key processes in reactive transport models could lead to significant error in prediction of reaction rates, consistent with a well recognized discrepancy between field and laboratory measured reaction rates³. Resolution of this discrepancy would provide better predictive capability to assess the fate of contaminants, carbon, and nutrients in the vadose zone.

¹Brantley and White (2009) Rev. Mineral. Geochem. 70: 435-484.

²Harrison et al. (2013) Environ. Sci. Technol. 47: 126-134.

³White and Brantley (2003) Chem. Geol. 202: 479-506.