The effect of pH and ionic strength on sorption of PFOS using soils with different TOC

Detailed knowledge on the fate and behaviour of PFAS in soils is one of the priority research topics recommended by the Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) in the U.S.A. The sorption is a critical process in controlling the fate and transport of PFAS in the environment (Weber et al., 1991; You et al., 2010). The sorption of PFOS onto soil has been reported to be influenced by various soil properties with total organic carbon (TOC) or soil organic matter (SOM) frequently reported to be the primary influencing factor. The sorption of PFOS was found to increase with TOC content in sediments (Higgins and Luthy, 2006) and in soils (Chen et al., 2013). This was attributed to the hydrophobicity of PFOS (Higgins and Luthy, 2006). Similarly, the sorption affinity of soils for PFOS decreased after removal of SOM (Qian et al., 2017). A sorption study of PFOS onto six soils (Wei et al., 2017) showed that Freundlich Kf was positively correlated with soil TOC as well as Al₂O₃ and Fe₂O₃ contents. However, there are studies that showed no correlation between sediment TOC content and sorption capacity of PFOS (Becker et al., 2008). At the same time, the sorption of PFOS by pure minerals was investigated to exclude the organic matter content effect. For instance, the sorption of PFOS on boehmite (AlOOH) was found to decrease with increasing solution pH and the addition of electrolytes (NaCl and CaCl₂). This was attributed to increases in ligand exchange and decreases of electrostatic interactions when pH increased, compression of the electrical double layer, and competitive adsorption of Cl-when electrolyte was added (Wang et al., 2012). Similarly, the sorption of PFOS onto Ottawa sand, goethite, kaolinite, and iron rich sand was investigated by Johnson et al. (2007). The sorption capacity showed linear increases with increasing initial concentrations and decreases with increases in pH. This demonstrated that electrostatic interactions played a role when organic carbon was absent. Chen et al. (2009) found the addition of phosphate reduced sorption of PFOS by soils (Qian et al., 2017), which was attributed to increases of negative charge on soil surfaces. The influence of cations and pH on sorption of 14 PFAS (including PFOS) on one organic rich soil was investigated (Campos Pereira et al., 2018). The sorption was found to inversely related to both pH and SOM bulk net charge. There is still a need to investigate influences of TOC, pH and ionic strength on sorption of PFOS by soils to provide more realistic information on the fate and behaviour of PFOS or similar PFAS. This study investigated the sorption of PFOS by eight soils varying in TOC at different pH and ionic strength conditions to reveal the mechanism and influencing effects. (author)