Nano-scale Characterization of Microprecipitates formed by Bacteria

During ureolysis-driven microbially induced calcium carbonate precipitation (MICP), ureolytic bacteria generate carbonate and hydroxyl ions which increase alkalinity of the cell-containing medium. In the presence of calcium, calcium carbonate can precipitate out of solution. Precipitates formed by MICP can be used in a variety of environmental applications as biological cement to seal leakage pathways of geologically stored carbon dioxide or to strengthen soil/sand. MICP also has potential in reducing the spread of heavy metal groundwater contaminants such as strontium and barium.

The micro-scale interactions taking place between the bacteria and the biominerals affect MICP in many ways including directing the biomineral crystallinity and polymorphism. Different polymorphs have different thermodynamic stabilities and can thus affect the long-term stability of the biocement seal. It is therefore essential to study MICP at biologically relevant scales to investigate and control the biological effect on precipitation.

We use a droplet-based microfluidics platform to generate individual precipitates ≤ 5 microns in diameter. The precipitation process begins with the formation of amorphous calcium carbonate, followed by the development of the crystalline form vaterite at the expense of ACC. Precipitates are visualized in-situ using confocal laser scanning microscopy (CLSM) and then prepared for ex-situ electron- and X-ray- based analyses using a novel method for preparing thin sections of precipitates using Focused Ion Beam (FIB). Preciptiates are characterized using energy dispersive spectroscopy (EDS), and near edge x-ray absorption fine structure (NEXAFS).

Our data show that precipitate morphology of microprecipitate aggregates is system specific, but crystallinity is uniform. Absorbance spectroscopy data suggest a difference in crystallinity between microprecipitate aggregates and cell-associated precipitates. Organic-mineral interactions that drive crystal polymorph selection could be leveraged into control parameters for large scale applications of these different polymorphs.