Long-term sustainability of microbial-induced CaCO₃ precipitation in aqueous media

Microbially induced CaCO₃ precipitation (MICP) via urea hydrolysis is an emerging technique for soil amelioration, building materials rehabilitation and pollutants sequestration amongst other various environmental applications. The successful application of MICP requires the sustainability of the precipitated CaCO₃; to which the fate of ammonia, the main by-product of ureolysis, is potentially significante. Ammonia volatilization and biological ammonia oxidation both induce a pH decrease, which, in turn, might cause CaCO₃ dissolution. To examine the potential effect of accumulated ammonia on precipitated CaCO₃, we conducted a long-term MICP batch experiment, using environmental enrichment cultures of ureolytic bacteria. Here we show that CaCO₃ precipitation was completed within 15-27 days, along with a rise in ammonium concentration. Following completion of ureolysis and precipitation, ammonium concentrations decreased, leading to a pH decrease. About 30 days after precipitation was completed, as much as 30% CaCO₃ dissolution, was observed. A two-step model, describing urea hydrolysis followed by the removal of ammonia from the precipitation solution, predicted CaCO₃ dissolution due to ammonia volatilization. We suggest that ureolytic MICP might result in ammonia volatilization, leading to significant CaCO₃ dissolution. These results provide basic insights into the sustainability of ureolytic MICP and should further encourage removal of the accumulated ammonia from the treated site.