No Discrepancy in Solid–Liquid Distribution of Perfluorooctanoic Acid Between Field-Contaminated and Lab-Spiked Soils

Abstract

Risk assessment of per- and polyfluoroalkyl substances (PFASs) requires accurate data on their fate in the environment. Current soil studies are generally based on short-term adsorption tests in soil spiked with PFAS, with limited attention to long-term reactions after that spiking (ageing) or to differences in solid–liquid partitioning between spiked and field-contaminated soils (field to spike). This study addressed both effects with a focus on perfluorooctanoate (PFOA), thereby using carrier-free ¹⁴C-labelled PFOA to discriminate the spiked from the field-originating PFOA. Short-term (48 h) adsorption of trace ¹⁴C-labelled PFOA in soils suspended in 0.01 M CaCl₂ indicated linear sorption; the PFOA distribution (K_D) values ranged from 0.2 to 46 L kg⁻¹ (median 2.2 L kg⁻¹) in 91 soil samples and correlated (p < 0.001) mainly with soil organic carbon (r = +0.65). Three soils were incubated up to 6 months after PFOA spiking. The desorption K_D values were only 1.7–2.8-fold higher than 48 h adsorption K_D values; these factors increased by ageing but plateaued 2–4 months after spiking. Field-contaminated soils were collected (n = 21, 0.5–1100 μg PFOA kg⁻¹). The PFOA desorption K_D was almost zero in field-contaminated soils with continuous fresh deposition and in soils with exceptionally high total PFAS concentrations (21000–53,000 μg kg⁻¹), the latter suggesting the formation of micelles facilitating desorption. In most other soils, PFOA desorption K_D values were similar to or maximally 1.6 times higher than corresponding ¹⁴C-PFOA adsorption K_D values measured in the same soils. Data suggest that PFOA adsorption is generally reversible and that small PFOA ageing effects observed in laboratory conditions at trace PFOA levels do not even occur in field conditions.