MINERALOGY OF PER-AND POLYFLUOROALKYL SUBSTANCES (PFAS) CONTAMINATED SOILS FROM AGRICULTURAL FIELDS IN SOCIETY HILL, SOUTH CAROLINA

Per-and polyfluoroalkyl substances (PFAS) are also known as ‘forever chemicals’ due to their chemical stability related to strong carbon-to-fluorine bonds. PFAS are used in fire-retardant foams, textiles, and food containers. PFAS are pervasive environmental pollutants found from metropolitan runoff to the extreme reaches of the Arctic. PFAS are toxins to human health. Agricultural fields treated with biosolids as soil supplements with a high PFAS content occur throughout the United States. Recent studies suggest a connection between clay minerals (e.g., kaolinite, hydroxy-interlayer vermiculite, and illite), oxyhydroxides (e.g., goethite and gibbsite), and PFAS. These clays and oxyhydroxides contain adsorbed hydrated cations with pore-mineral surfaces likely influenced by pH and redox conditions and thus have the potential to adsorb/desorb PFAS.

We examined 89 samples from 2 drill sites (~18 m) in spray application fields where wastewater from a textile factory was applied. This study determined the: (1) degree of contamination of PFAS performed by the U.S. Environmental Protection Agency (EPA), (2) weight percent (wt.%) minerals using X-ray diffraction (XRD) Rietveld analysis, (3) wt.% of extractable crystalline (E-Xtal) Fe and Al using an acid technique (EPA), and (4) correlations of wt.% Fe-goethite versus wt.% Fe E-Xtal and wt.% Al-gibbsite versus wt.% Al E-Xtal.

Results reveal high PFAS contamination in the top 3 meters with an increased abundance of goethite, gibbsite, and E-Xtal Fe and Al. The XRD analysis shows that kaolinite is a major clay mineral constituent, with gibbsite, goethite, illite, and vermiculite occurring in lesser amounts. The linear correlation coefficient (R²) between wt.% Fe-goethite versus wt.% Fe E-Xtal in sites 1 and 2 are 0.69 and 0.50, respectively, while the R² of wt.% Al-gibbsite versus wt.% Al E-Xtal in sites 1 and 2 are 0.50 and 0.23, respectively. The higher correlations between Fe-goethite and Fe E-Xtal than Al-gibbsite and Al E-Xtal suggest the acid extraction method is better for determining Fe-content than Al-content. Given differences in the zero points of charge for the various minerals, we expect PFAS adsorption and desorption behavior to be closely related to mineral abundances and in situ pore water redox and pH conditions.