Biogeochemical Model for Soil Improvement by Denitrification

A biogeochemical model describing the rate at which calcium carbonate (${\mathrm{CaCO}}_3$) is precipitated from the pore water and biogas (carbon dioxide and nitrogen gas) is generated by dissimilatory reduction of nitrate (denitrification) through microbially induced desaturation and precipitation (MIDP) has been developed. Both ${\mathrm{CaCO}}_3$ precipitation and desaturation via biogas formation can improve the static and cyclic mechanical properties of granular soil. ${\mathrm{CaCO}}_3$ precipitation can improve the static and cyclic stiffness, shear strength, and volume change characteristics of granular soil. Desaturation via biogenic gas generation suppresses excess pore pressure development and thereby improves the cyclic shear resistance of granular soil. MIDP represents the combined effect of these two mechanisms. Effective implementation of MIDP for ground improvement demands quantitative understanding of the rate at which both mechanisms occur. The biogeochemical model developed herein is an upgrade of the model presented in earlier research; it includes additional features, such as inhibition terms, an expanded number of organic substrates, and a greater number of chemical constituents. It predicts the rate of ${\mathrm{CaCO}}_3$ precipitation and gas production by integrating stoichiometry, thermodynamics, microbial growth kinetics, and chemical reaction kinetics for a continuously stirred batch reactor. The model was calibrated using results from laboratory test columns. Sensitivity analyses conducted using the calibrated model identified a molar ratio for acetate:calcium:nitrate of $0.9:1.0:1.0$ as the preferred ratio for maximizing ${\mathrm{CaCO}}_3$ precipitation and gas generation while avoiding excess residuals in the modeled batch reactor system.