Insights into the synergistic effects between deposited Fe oxides and dissolved organic matter in influencing perfluorooctanoic acid transport in saturated porous media

Perfluorooctanoic acid (PFOA) transport in the subsurface environment is relevant to drinking water safety, while the compounding effects of soil components on PFOA migration are poorly understood. Laboratory miscible-displacement experiments were conducted using saturated sand columns to explore how metal oxide surfaces and dissolved organic matter (DOM) jointly affect PFOA transport in porous media. Retardation factors indicated that Fe oxide coating inhibited PFOA migration due to electrostatic interaction. However, PFOA recovery rates changed insignificantly, decreasing by less than 4 % when the proportion of Fe oxide-coated sand reached 50 %. DOM (1 mg/L humic acid) in the pore water slightly decreased PFOA recovery rates (by about 10 %) in quartz sand, indicating the effect of hydrophobic interaction on PFOA migration. When the PFOA solution containing 1 mg/L humic acid was injected into the column containing Fe oxide-coated sand, PFOA recovery was significantly decreased by nearly 20 %, and the retardation factor was more than doubled. This could be attributed to the stronger hydrophobic effect provided by the higher DOM deposition on the Fe oxide surface. These results, supported by SEM-EDS, zeta potential, and model fitting data, highlight the microscopic mechanisms by which interactions between metal oxides and DOM influence PFOA transport. However, this inhibitory effect disappeared at higher humic acid concentrations (20 mg/L), indicating the risk of PFOA re-migration when the DOM concentration greatly exceeds the adsorption capacity of the media for it. The findings of this work have implications for predicting or controlling the environmental risks of PFOA in soil and groundwater.