Effect of particle size on migration and retention of bacteria in sand and its biomineralization

Bacterial cells are widely accepted as nucleation sites for calcium carbonate precipitation in biomineralization based on the Microbially Induced Carbonate Precipitation (MICP) process. For MICP-based in-situ biotreatment, the first problem to be solved is how to introduce and retain the bacterial cells in the soil, which involves the migration and retention of bacterial cells during the biogrouting process. Soil particle size, a key factor in determining pore throat size, can have a significant effect on the migration and retention of bacterial cells in the soil and therefore on biomineralization. To investigate the effect of particle size on the migration and retention of bacterial cells in sand and its biomineralization, two sets of tests were carried out in this study, including percolation tests and sand column treatment tests. Soil urease activity (defined as urease activity per unit mass of soil) and calcium carbonate content of the biomineralized sand were measured to comprehensively assess the migration and retention of bacterial cells in the sand. The results indicate that sands with a particle size smaller than 0.25 mm would inhibit the migration of bacteria in the sand, resulting in a nonuniform distribution of precipitated calcium carbonate and a low strength enhancement of biomineralization. On the other hand, sands with a particle size larger than 1.18 mm are unfavorable for retaining bacterial cells in the sand, resulting in low calcium conversion efficiency. Meanwhile, particle size would also affect the formation of effective calcium carbonate through interparticle contact number and interparticle pore size, and thus biomineralization.