Bio-sealing performance evaluation using microbially induced and enzyme-induced biopolymer formation

Abstract

Leakage in waterfront or water-retaining structures poses risks to the safety of structures, and it is therefore one of the major issues in geotechnical engineering practices. “Bioclogging” refers to a reduction in the hydraulic conductivity of soils and rocks due to microbial activities and by-products. Previous studies have investigated the feasibility of bioclogging through laboratory column experiments. However, the mechanism of bioclogging on a microscale level remains unclear. In addition, limited studies have been conducted on improving the efficiency of bioclogging. Therefore, this dissertation aims to (a) attain a better understanding of bioclogging mechanisms at the micro-scale, (b) examine the factors affecting the bioclogging efficiency in coarse sands, and (c) propose new methods to overcome the limited applicability of microbial treatment in fine-grained soils and to enhance bioclogging durability under starved conditions. The pore-scale patterns of biopolymer formation are analyzed based on images acquired via microfluidic chip experiments. After bacterial cells are attached to a solid surface in the chip, the cells begin to produce an insoluble biopolymer called dextran. This cell-driven formation results in a smaller size of dextran and takes a longer time compared to the cell-free formation ⸻ microbially induced biopolymer formation (MIBF). In contrast, the enzyme forms a biopolymer soon after the injection of enzyme/sucrose mixed solution ⸻ enzyme-induced biopolymer formation (EIBF), ⸻ and produces a larger dextran size than the cell-driven dextran. The effects of several factors, such as particle size, nutrient pH, and biogenic gas generation on engineered bioclogging are investigated via a series of column experiments. The results reveal that fermentation-based bacterial biopolymer formation can reduce the hydraulic conductivity of coarse sand by three orders of magnitude or by 99.9% in controlled environments, which implies a reduction in hydraulic conductivity to the level of silts. The method for producing biopolymers with enzymes but without cells is explored as micron-sized bacterial cells hardly thrive in fine soils with a pore size of micrometers. The enzyme “dextransucrase” is extracted from a series of treatments including sonication, centrifugation, and syringe filtering. Using the extracted cell-free enzyme solution, the EIBF method is optimized via a series of batch experiments to determine the sucrose concentration, sucrose-to-enzyme mixing ratio, and kinetics. The EIBF shows a higher efficiency than MIBF at decreasing the hydraulic conductivity, which proves the feasibility of using the enzyme “dextransucrase” for bioclogging. Finally, the effect of bentonite-biofilm interactions on the durability of biofilm-induced bioclogging is examined. The results indicate that the exploitation of bentonite-biofilm aggregations by injecting bentonite suspensions can significantly enhance the bioclogging durability under nutrient-poor conditions. These results provide fundamental insights into the bacterial biopolymer formation mechanism, its effect on soil permeability, and the potential for engineering bacterial clogging in the subsurface.