Escherichia coli migration in saturated porous media: Mechanisms of humic acid regulation.

The regulatory behavior of humic acid (HA) on the migration of Escherichia coli (E.coli) in saturated porous media has garnered considerable research interest. Although prior studies have confirmed that HA indeed facilitates the migration of E. coli in saturated porous media, investigating the migration process and regulatory mechanisms at the microscale remains challenging. This study compared the differences in the migration behavior of E. coli in saturated porous media under conditions with and without HA, revealing the dynamic mechanism by which HA regulates microbial migration through the "bacterium-medium-solution" triple interface interaction. The results indicated that E. coli achieves the transition of the "run-tumble" movement pattern (run ≈ 1 s, tumble ≈ 0.1 s) through flagellar morphological regulation, thus completing directed migration in a complex pore network. The addition of HA significantly enhanced the migration rate of E. coli, with an increase of at least 5 %. For the bacteria, HA induced the restructuring of lipopolysaccharides on the bacterial surface, altered the surface Zeta potential of the bacteria, and promoted the formation of stable hetero-aggregates between bacteria and HA. At the medium interface, HA modifies the surface charge of the medium, regulates pore structure, and increases hydrophilicity through the adsorption-desorption mechanism. In the solution system, the dissociation characteristics of HA's carboxyl and phenolic hydroxyl groups dynamically regulated the solution's ionic strength and pH value, creating a chemical microenvironment suitable for bacterial migration. This study systematically revealed the multi-dimensional mechanisms by which HA regulates microbial transport through molecular interface engineering. It provides theoretical support for establishing predictive models of pathogen migration in groundwater systems and offers important guidance for optimizing microbial control strategies in water treatment processes.