The effects of soil properties and co-contaminants on sorption of perfluorooctane sulfonate (PFOS) in contrasting soils

Sorption plays an important role in controlling the fate and behaviour of per- and polyfluoroalkyl substances (PFAS) in soils. However, there is no consensus on the influence of key soil properties on sorption of perfluorooctane sulfonate (PFOS) in the current literature. In particular, the effects of soil mineral components on sorption have not been fully investigated. In this study the sorption isotherms of PFOS under different electrolytes (Na+ and Ca2+) was investigated using eight contrasting soils that varied in pH, total organic carbon (TOC) content, clay content and other physico-chemical properties. In addition the sorption behaviour of PFOS was studied at different pH values (3-9) and ionic strengths (0.03 and 0.3 M NaNO3) to determine the role of hydrophobic and electrostatic interactions on PFOS sorption. The effects of possible co-contaminants, lead (Pb) (20.8 mg/L Pb2+) and dissolved organic carbon (DOC, 11.5 mg/L as humic acid) and perfluorooctanoic acid (114 mu g/L PFOA) on sorption of PFOS were also investigated. A strongly significant linear relationship (p<0.01) between TOC and the amount of PFOS sorbed was observed irrespective of the background electrolyte index cation, Na+ or Ca2+. The presence of DOC, and other common co-contaminants like PFOA, and Pb2+ in solution generally increased PFOS sorption by the soils. Sorption of PFOS at different ionic strengths and pH demonstrated that the sorption was dominated by hydrophobic interactions when the TOC content was high, but electrostatic interactions dominated at lower pH values and in soils with smaller TOC contents. This study demonstrates the role of both organic matter and surface chemical properties of soils as key factors influencing PFOS sorption. The use of multiple soil properties to explain the sorption is required. (C) 2020 Elsevier B.V. All rights reserved.