Assessing the Kinetics and Pore-Scale Characteristics of Biological Calcium Carbonate Precipitation in Porous Media using a Microfluidic Chip Experiment

Biomineralization through microbially or enzymatically induced calcium carbonate precipitation (MICP/EICP) by urea hydrolysis has been widely investigated for various engineering applications. Empirical correlations relating the amount of mineral precipitation to engineering properties like strength, stiffness, or permeability show large variations, which can be partly attributed to the pore-scale characteristics of the precipitated minerals. This study aimed to gain insight into the precipitation kinetics and pore-scale characteristics of calcium carbonate minerals through time lapse imaging of a transparent microfluidic chip, which was flushed 10 times with a reactive solution to stimulate EICP. An image processing algorithm was developed to detect the individual precipitated minerals and separate them from the grains and trapped air. Statistical analysis was performed to quantify the number and size distribution of precipitated minerals during each treatment cycle and the cumulative volume, surface area, bulk precipitation rate, nucleation rate, and supersaturation were calculated and compared with a simple numerical model and more complex theory on precipitation kinetics. The analysis showed that results were significantly affected by the assumed particle shape. The supersaturation, which controls the crystal nucleation and growth rates, was shown to be a function of the hydrolysis rate, the kinetic order and growth rate constant, and available surface area for mineral growth. Possible explanations for observed discrepancies between observations and theory, including diffusion limitations, the presence of inhibiting compounds, local pore clogging or observation bias, and limitations of the methodology, are discussed.