Regulating the Process of Microbially Induced Calcium Carbonate Precipitation through Applied Electric Fields: Evidence and Insights Using Microfluidics

Abstract

Nonuniform cementation challenges may arise when employing microbially induced calcium carbonate precipitation (MICP) technology to treat problematic soils. Electrokinetic methods exhibit the potential to regulate the MICP process by driving negatively charged urease-producing bacteria and promoting the migration of reactant ions. However, the current understanding of bacterial behavior and the characteristics of CaCO3 precipitates under the influence of direct current (DC) electric fields remains unclear. In this study, we designed and fabricated microfluidic chips to simulate sandy soil matrices. By monitoring the real-time MICP reaction process in the microfluidic porous medium with and without a DC electric field through an optical microscope, we analyzed the impact of the electric field on the behavior of bacteria at different locations (i.e., attached and suspended bacteria) and on the morphology and distribution of CaCO3 crystals. Results show that initially injected bacteria adhered to the inner pore surfaces due to interfacial cohesion, whereas subsequently injected bacteria were predominantly suspended in the pore solution and driven toward the anode by the electric field, where they aggregated. Furthermore, the majority of Ca2+ precipitated near the cathode, resulting in larger-sized crystals, and a smaller quantity of Ca2+ precipitated at the anode, generating smaller-sized crystal particles. This observation highlights the crucial role of Ca2+ movement in determining the distribution of CaCO3. Under the influence of a DC electric field, the final precipitated CaCO3 crystals exhibit nonuniform sizes, with an overall smaller average size and more complex crystal morphologies. Our results provide the underlying insights to guide the regulation of the MICP process through applied electric fields.