Natural colloids can alter the bacterial surface biomolecules and interfacial energies affecting the transport of bacteria in unsaturated porous media.

Pathogenic bacteria contamination caused by reclaimed water irrigation poses significant threats to arid and semi-arid ecosystems. However, the impact and mechanisms of the complex environment in the vadose zone on the fate and transport of bacteria remain unclear. The individual and combined effects of representative natural colloid (humic acid (HA) or calcium carbonate (CaCO3)) on the transport of Pseudomonas aeruginosa (P. aeruginosa) in unsaturated porous media under various pH values (7.0, 8.5, and 9.5) and cation types (Na+ and Mg2+) were investigated using column experiments and characteristic analysis. The results indicated that the presence of CaCO3 inhibited bacterial transport (with average migration capacity from 0.429 to 0.317) under all examined solutions, due to its decreased energy barriers (from 8.47 to 7.78 kT) between bacteria and solid-water interface (SWI) and the concentration of bacterial ATP decreased by 48.1 %. However, HA disrupts the original migration pattern of bacteria observed in the presence of CaCO3, resulted in significant reductions in amide Ⅰ and protein concentrations to 1.03 % and 2.98 %, respectively. The intensified energy barriers (11.45 kT) between bacteria and SWI greatly increased migration capacity to 0.549. Notably, HA-mediated transport of bacteria was highly dependent on solution chemistry. Specifically, HA promoted bacterial transport (with average migration capacity from 0.365 to 0.380) in MgCl2 and neutral NaCl solutions owing to the competitive retention and steric hindrance effects, while inhibiting bacterial transport (with average migration capacity from 0.555 to 0.182) in alkaline NaCl solutions due to the formation of bacteria aggregates. Moreover, the bacterial transport behavior was further elucidated through the two-site kinetic attachment model and colloid filtration theory (CFT). Findings of this study offer theoretical support for understanding pathogenic bacterial migration patterns and mechanisms in soil-water environments.