Transport and retention of perfluorooctanoic acid (PFOA) in natural soils: Importance of soil organic matter and mineral contents, and solution ionic strength

Effects of soil organic matter and mineral contents, and solution ionic strength on the transport of perfluorooctanoic acid (PFOA) in different types of soils (i.e., desert soil, black soil, and red soil) were systematically investigated with a set of laboratory column experiments. The retention of PFOA in the soils under two solution ionic strengths conditions (1.0 mM and 10.0 mM CaCl2) followed an order of red soil > black soil > desert soil (expect red soil in 10 mM CaCl2). PFOA retention in the black soil (soil organic carbon content 2.57%) was higher than that in the desert soil (soil organic carbon content: 0.05%). In addition, PFOA also showed higher mobility in treated black soils with lower organic carbon contents. These results suggest that the hydrophobic interaction is a key mechanism governing PFOA retention and transport in soils. Besides, more PFOA retention in the red soil (zeta potential: 7.25 +/- 0.10 mV and 14.80 +/- 0.20 mV) than the desert soil (-15.70 +/- 0.10 mV and -9.11 +/- 0.10 mV) was observed, suggesting that electrostatic interaction may be another important mechanism for PFOA retention and transport in soils. Increasing ionic strength slightly increased the retention of PFOA in negatively charged desert and black soils, but significantly decreased the retention of PFOA in the positively charged red soil. Findings of this study point to the importance of soil physiochemical properties to the fate and transport of PFOA in natural soils.