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

Abstract

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.

Experimental Methods and Imaging for Enzymatically Induced Calcite Precipitation in a Microfluidic Cell

“…The pore throats have a size of 125 μm and the pore bodies a diameter of 500 μm. This is comparable to the geometric features of previous studies, like in Wang et al (2019) or Kim et al (2020) where the size of the pore throats range from below 50 to 140 μm. The inlet at the bottom left is connected with two 2.5 mL glass syringes (S 1 , S 2 ) guided by two, one for each, midpressure pumps CETONI neMESYS 100 N which generate the flow.…”

“…The concentrations we used to prepare the reactive solutions are based on similar investigations on EICP, like in the work of Kim et al. (2020) and Feder et al. (2020).…”

“…In the work of Kim et al. (2020), the volume of individual crystals is estimated by assuming the shape of the crystals to be either cylindrical or semispherical. From the area observed by optical microscopy ( A ), an equivalent radius ( r eq ) is calculated: …”

“…This induces a back-pressure in order to reduce the risk of bubble formation during the experiment. Air bubbles can lead to difficulties in image postprocessing, as shown in the work of Kim et al (2020), and are likely to disturb the pressure measurements. The tubes for the outlet have an inner diameter of 0.75 mm to avoid clogging.…”

“…The volume of individual crystals is then estimated based on the equivalent radius and the height of the microfluidic cell. However, in the work of Kim et al (2020), the shapes are just hypothetical and have not been validated by three-dimensional imaging techniques. Following the same approach, other shapes like spherical or spheroidal shapes are potentially possible as well.…”