Clay-bacteria interaction: Effect of bacterial cell density on sedimentation behavior and fabric map of kaolinite clay

Clay-bacteria interactions are ubiquitous and the presence of bacteria has a pronounced effect on the mechanical, chemical, and electrical behaviors of clays. However, the role of microscale clay-bacteria interactions to the macro-scale engineering behaviors and properties of clays remains poorly understood. This study presents changes in the microstructure of kaolinite in association with bacteria and the resulting sedimentation behavior of bacterium-associated kaolin suspensions. Sedimentation tests were performed using suspensions composed of kaolin and model bacteria Shewanella oneidensis MR-1 while varying the fluid pH and cell density. The results reveal that kaolinite presents face-to-face aggregated microstructures with the presence of bacteria, attributable to sharing and/or exchange of cations between kaolinite and bacterial cells. Meanwhile, at a pH value lower than the isoelectric point of the kaolinite edge surfaces, the clay-bacteria association appears less pronounced comparing to the higher pH, owing to the competition between negative (kaolinite faces and bacteria surfaces) over positive (kaolinite edges) charges. The fabric change caused by kaolinite-bacteria association accelerates kaolin settling and reduces the final void ratios. A fabric map for bacterium-asssociated kaolinite is suggested based on the sedimentation behaviors and scanning electron microscopy observations. This study advances the understanding of particle-level clay-bacteria interactions and the sedimentation behavior of clays in suspensions with vigorous microbial activities.