Influence of different types of humic substances on the adsorption behavior of PFOA in soils: Experimental insights and spectral characterization

The adsorption behavior of perfluorooctanoic acid (PFOA) in soils is critical for understanding its environmental transport. However, the effects and mechanisms of humic substances (HS) as an organic soil amendment on PFOA adsorption in soils are still unclear. This study investigated the interactions between HS, particularly humic acid (HA) and fulvic acid (FA), PFOA and soil during the PFOA adsorption process in soil environment. The adsorption capacities of PFOA followed the order: Soil > Soil + HA > Soil + FA > HA > FA. HS, with their negative charges, hindered PFOA adsorption by occupying surface adsorption sites. Fluorescence quenching experiments confirmed HS-PFOA complex formation. PFOA preferentially bound to humic acid-like and fulvic acid-like substances. The primary adsorption sites in both soil and HS were -OH and C-O-C groups. Although HS formed complexes with PFOA, they reduced the overall adsorption capacity in soil. HA provided more hydrophobic interaction sites for PFOA than FA, and FA, with a higher negative charge, exerted stronger electrostatic repulsion, leading to a more significant reduction in PFOA adsorption. This study highlights the role of different HS in PFOA soil adsorption and offers insights for soil remediation and risk assessment of PFOA.