Calcium Carbonate Precipitation Visualization on a Lab-on-a-Chip Model With Spectral Induced Polarization Monitoring

Abstract

Calcium carbonate (CaCO₃) is ubiquitous in natural and engineering processes. Its dynamic morphological evolution is critical in geological carbon sequestration, environmental remediation, and geomaterial strengthening, but remains insufficiently visualized and monitored at the particle scale. In this study, a microfluidic model with a 20× optical objective lens was developed to visualize the dynamic growth process of inorganically generated CaCO₃ crystals. Complementary spectral induced polarization measurements were carried out to monitor the content and morphology of CaCO₃ crystals. Dispersed CaCO₃ precipitates formed during the early stage, and the mean crystal size increased to 3.4 μm at 2.0 PV. Afterward, precipitates accumulated, especially at the interface between Na₂CO₃ and CaCl₂ solutions, while the morphology and size of the individual crystals were masked by the accumulation. The measured imaginary conductivity increased monotonically as the amount of the dispersed precipitates increased until 2.0 PV, and then declined as clusters formed. When dispersed precipitates dominated before 2.0 PV, normalized chargeability was more strongly correlated with CaCO₃ volumetric content than with estimated specific surface area, because volumetric content provided a more robust indicator of the abundance of polarizable material, while conductivity normalization reduced the influence of pore-fluid conductivity on the measured polarization magnitude. Relaxation time followed the Schwartz relationship for discrete precipitates but deviated once aggregates and the precipitation band formed because their visible dimensions no longer represented the effective polarization length scale. These observations provide particle-scale evidence for the dynamic evolution of CaCO₃ precipitation and clarify the applicability of SIP-based characterization.