Conceptual Geochemical Model for Sorption and Desorption of PFAS

The unique properties and behaviour of per- and polyfluorinated alkyl substance (PFAS) in the environment pose challenges to assessment of fate and transport. PFAS compound sorption and desorption is affected by the hydrophilic and hydrophobic properties of these compounds. Studies show that organic carbon (OC) content may not be the dominant factor that affects sorption. Alumino-silicate and iron oxide mineral present other significant sorption sites of PFAS compounds.

This is an important consideration in Australia, where soils are old and well leached, with low OC and carbonate content and enriched in iron and aluminium minerals. Consequently pH can play a significant role in the sorption and desorption of PFAS compounds as unlike of OC, iron and aluminium mineral sorption properties are affected by pH, with these minerals having a point of zero charge at specific pH, below which they are anion attracting, while they attract cations above the point of zero charge. Given that most PFAS compounds are anions in solution this aspect has particular significance in the Australian spoil context.

Considering these unique Australian soil properties a Conceptual Geochemical Model (CGM) was developed that considered the effects that soil mineralogy, pH and recharge on the fate and transport of PFAS compounds through the unsaturated zone to the saturated zone and the subsequent transport in the groundwater system to the a receptors. The CGM considers the geochemical dynamics in the unsaturated zone in the context of desorption, while consideration is given to the equilibrium behaviour in the saturated zone.

The conceptual mode was applied and verified using data from a coastal zone airport where PFAS impacts migrated from a former fire training ground through a freshwater groundwater system to a marine receptor. The study found that PFAS fate and transport was more akin to inorganic solute transport than organic compounds.