Dynamics of Mineral Precipitation in Diffusion Controlled Mixing Zones

Stimulating in situ precipitation of minerals to immobilize metal contaminants is a promising approach for subsurface remediation. However, predicting and controlling the spatial/temporal distribution of reactant mixing and precipitates remains a challenge. The formation of precipitates within reactant mixing zones can modify local and volume-averaged porosity and can lead to assymmetric and transient modifications in solution chemistry across the precipitation zone. This in turn impacts the local distribution, composition and morphology of the precipitated phases. The chemical and physical processes involved are tightly coupled and non-linear, presenting challenges for model simulations at multiple scales. We have conducted experiments in double diffusion cells where reactants - calcium and either carbonate or phosphate - mix in gel or granular media through diffusive transport. The objective is to observe and simulate transients in mineral precipitate distributions that can influence: 1) long-term outcomes of reactions in the affected region, and 2) the ability of models that employ volume averaging to predict changes in local properties of the system, such as extent, rates and products of reactions. Analysis of the transient nature of the precipitation events was aided by a simple digital time-lapse photography and spectrum decomposition method. In the diffusion cells precipitation band formation occurs as expected (analogous to Liesegang patterns). The general results for the Ca2+/CO32- system are distinctly different from the Ca2+/PO43- system. In the phosphate system, nucleation and growth of precipitate occurs in multiple narrow bands, which are composed of relatively densely packed, fine grained material. In the carbonate system CaCO3 crystals are distributed within a broader region and individual crystals are more widely dispersed. Differences indicate the relative influence of rates of nucleation, growth and diffusive transport of reactants.