Biogeochemical Cycling of Selected PFAS Compounds in Tennessee Loring Silt Loam, Insights from in situ-ATR-FTIR Probes

 

Polyfluoroalkyl Substances (PFAS) is a general class of organic compounds used for various industrial and household purposes including as a fire retardant.  Because of their extreme structural stability under various conditions, PFAS compounds are known to persist in the environment causing toxicity to higher animals including humans. Both US EPA and EU set the maximum contaminant levels of PFAS in water ranging from 10-100 ppt.  Soil is an important source and sink for PFAS compounds and adsorption is known to be an important process regulating the fate of these compounds.  Thus, it is important to evaluate adsorption mechanisms of PFAS compounds on soil.  Although various research studies focused on PFAS adsorption on soil, the application of detailed spectroscopic probes to isolate the mechanisms is rare.  Here we propose to examine adsorption mechanism of selected PFAS compounds (PFOS: Perfluorooctanesulfonic acid and PFOA: Perfluorooctanoic acid) on a Tennessee native soil, Loring silt loam (Oxyaquic Fragiudalfs) using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic probes under various solution properties. Since soil mineralogy plays a dominant role in surface interactions with organic molecules, PFOS and PFOA adsorption mechanisms on kaolinite, an important mineral in Loring silt loam, have been studied as well.  Our initial findings suggested that PFOS makes an outer-sphere surface complex with Loring silt loam at pH 5 and ionic strength of 0.01 M NaCl.  When inorganic P (Pi) was added, infrared (IR) bands relevant to PFOS adsorption decreased and some evidence of IR bands pertaining to Pi appeared, thereby corroborating outer-sphere adsorption.  Adsorption of POFA on Loring soil indicated a similar outcome.  Our results will help understand the mechanisms of biogeochemical cycles of PFAS compounds in the soil environment.