Coupling Adsorption and Persulfate Oxidation Using FeS2-Zeolite for Efficient PFOA Removal

 Abstract

Per- and polyfluoroalkyl substances (PFAS) are widespread groundwater contaminants that are increasingly causing concern and prompting regulatory action due to their extreme persistence, mobility, limited biodegradability, and harmful properties. Perfluorinated compounds in particular, such as perfluorooctanoic acid (PFOA), one of the most common and most studied representatives of this class of PFAS, also show high resistance to chemical degradation and pose particular challenges for conventional remediation methods. Current treatment approaches are therefore based either on adsorptive removal from water without destruction or on energy-intensive processes that allow the molecules to be destroyed. Therefore, new low-energy methods are urgently needed. Integrated systems that combine pre-enrichment with efficient degradation under environmentally relevant conditions are therefore the focus of recent research.

In this study, FeS₂-zeolite composites were developed by immobilizing crystalline pyrite on a hydrophobic BEA-35 zeolite to achieve coupled in-situ adsorption and peroxydisulfate (PS) activation for PFOA degradation. FeS₂-BEA35 facilitated hydrophobic enrichment of PFOA within its porous structure with a K_D value of 1.8 × 10⁴ L/kg at a c_free of around 50 µg/L. The study on the mechanism illustrated that sulfate radicals (SO₄^•−) were predominantly generated through surface-mediated homolytic PS cleavage on the FeS₂-BEA35 surface, serving as the dominant species for PFOA degradation. Alongside sulfate radicals, the results showed the involvement of previously unrecognized Fe^IV=O²⁺ species, generated by the oxidation of Fe(OH)(H₂O)₅²⁺-PFOA complex by SO₄^•−. The Fe^IV=O²⁺ species acted as a secondary reactive species capable of abstracting electrons from the coordinated PFOA, thereby promoting decarboxylation and C-C bond cleavage. Although coexisting inorganic ions and natural organic matter reduced adsorption and degradation rates of PFOA, yet FeS₂-BEA35 still maintained strong affinity and reactivity. Fixed-bed column experiments started with an influent PFOA of 1 mg/L, designed to approximate continuous treatment conditions, demonstrated a high PFOA adsorption capacity of about 1.2 × 10³ mg/kg at a c_free of around 60 µg/L and sustained over 70 % removal efficiency under cyclic oxidation with low Fe leaching. The efficient PS utilization also confirmed dominant heterogeneous activation and excellent catalyst stability.

Therefore, FeS₂-BEA35 integrates efficient adsorption and durable catalytic reactivity, offering a promising platform for continuous perfluorocarboxylic contaminant remediation through interfacial enrichment and surface-mediated PS cleavage.