CaCO3 precipitation, transport and sensing in porous media with in situ generation of reactants.

Ureolytically driven calcite precipitation is a promising approach for inducing subsurface mineral precipitation, but engineered application requires the ability to control and predict precipitate distribution. To study the coupling between reactant transport and precipitate distribution, columns with defined zones of immobilized urease were used to examine the distribution of calcium carbonate precipitation along the flow path, at two different initial flow rates. As expected, with slower flow precipitate was concentrated toward the upstream end of the enzyme zone and with higher flow the solid was more uniformly distributed over the enzyme zone. Under constant hydraulic head conditions the flow rate decreased as precipitates decreased porosity and permeability. The hydrolysis/precipitation zone was expected to become compressed in the upstream direction. However, apparent reductions in the urea hydrolysis rate and changes in the distribution of enzyme activity, possibly due to CaCO3 precipitate hindering urea transport to the enzyme, or enzyme mobilization, mitigated reaction zone compression. Co-injected strontium was expected to be sequestered by coprecipitation with CaCO3, but the results suggested that coprecipitation was not an effective sequestration mechanism in this system. In addition, spectral induced polarization (SIP) was used to monitor the spatial and temporal evolution of the reaction zone.