Mechanisms That Control Biomineralization of Carbonates in Porous Media.

Biogeochemical reactions in soils and sediments can alter pore space geometry, and affect fluid residence time, solute concentration gradients, and subsequent biogeochemical reaction rates.  Of particular interest are biogeochemical reactions that involve carbonates, which can dominate pore space alteration in tropical soils, karst aquifers, and deep subsurface reservoirs.  Carbonate reactions kinetics have been extensively studied in batch systems, but not in the context of reactive transport where mixing is incomplete and mass transfer limitations affect reaction rates.  In this work, we evaluate the mechanisms that control the biomineralization of carbonates during reactive transport in a microfluidic pore network fabricated from a precision-etched silicon wafer.  The pore network represents a model of soil or groundwater sediments, and allows direct visualization and imaging of solute mixing, biomass growth, and chemical precipitation.  Carbonate precipitation was induced by allowing nitrate and acetate to mix through dispersion in the pore network in the presence of 20 mM Ca2+ and a strain of Pseudomonas stutzeri.  The P. stutzeri produced alkalinity during nitrate reduction and Raman spectroscopy results indicate calcite was formed.  Control experiments were performed at the higher alkalinity in the absence of any P. stutzeri, and in the presence of inactive P. stutzeri. The results will be presented and used to evaluate whether carbonate precipitation was caused by higher alkalinity, both higher alkalinity and microbial nucleation sites, or the combination of higher alkalinity, microbial nucleation sites, and P. stutzeri activity.