Phosphate-induced surface roughening suppresses bacterial transport in coral sand

Phosphate input is widely used to improve nutrient availability in coral sand soils on artificial islands, yet its effects on microbial transport remain unclear. This study investigated how phosphate accumulation affects Escherichia coli (E. coli) transport in coral sand. Phosphate treatment promoted Ca-P formation and transformed the coral sand surface into a P-enriched, roughened interface while largely preserving the bulk pore structure. In column experiments, bacterial breakthrough decreased from 77.8 ± 0.8% in untreated coral sand (CS) to 17.4 ± 0.1% in highly phosphate-treated coral sand (HCS), while the deposition rate coefficient increased from 0.722 to 5.67 h^-1. Enhanced retention persisted under varying ionic strength, grain size, and flow velocity. Batch adsorption and density functional theory calculations indicated that enhanced direct chemical affinity alone was unlikely to fully explain the observed immobilization. SEM, AFM, micro-CT, and roughness-corrected XDLVO analyses supported that phosphate-induced roughening created microscale trapping sites, lowered bacteria-collector energy barriers, and enhanced physical retention. Laterite colloids reduced bacterial mobility. These findings suggest that phosphate-induced surface reconstruction can limit downward bacterial penetration and potentially reduce microbial contamination risks to island groundwater. This mechanism supports predicting leaching risks under phosphate-loading scenarios and designing nutrient-input strategies that balance fertility improvement with groundwater protection.