G. Kadarkarai, B.K. Mayer, P.J. McNamara
Marquette University,
United States
Keywords: perfluorooctane sulfonic acid, iron-electrocoagulation, persulfate activation, extraction validation, floc immobilization
Summary:
Decoding PFOS Fate and System Design for Fe-Electrocoagulation Govindan Kadarkarai, Brooke K. Mayer, Patrick J. McNamara Department of Civil, Construction and Environmental Engineering, Marquette University, Milwaukee, WI 53233, United States. Perfluorooctane sulfonic acid (PFOS) remains among the most recalcitrant perfluoro alkyl substances (PFAS) in water treatment. We couple mechanistic diagnostics with extraction-based validation to resolve Perfluorooctane sulfonic acid (PFOS) fate in iron electrocoagulation (Fe-EC), with and without the addition of peroxomonosulfate (PMS) or peroxydisulfate (PDS), and define operating windows relevant to deployment. Under Fe–EC operation (2.08 mA cm⁻², 60 min, 250 mg L⁻¹ Cl⁻), ~80% PFOS was removed from solution (Figure 1). Reactive species including HO•, O₂•⁻, ¹O₂ , and SO₄•⁻ (when PMS or PDS was used) were detected; however, no measurable transformation products were observed, indicating a non-destructive, coagulation-dominated pathway. To test whether oxidant coupling improves outcomes, we compared Fe-EC to Fe-EC–PMS and Fe-EC–PDS (Figure 2). Despite broader radical formation, both PMS and PDS reduced net PFOS removal relative to Fe-EC, consistent with competition for Fe²⁺ and suppression of Fe(OH)x floc formation. Matrix effects were explored in the Fe-EC–PDS system. The presence of Cl⁻ and SO₄²⁻ supported PFOS capture, whereas CO₃²⁻, NO₃⁻/NO₂⁻, and HPO₄²⁻ inhibited removal likely via pH buffering and Fe-complexation, establishing practical constraints for real waters. Natural organic matter impacted performance asymmetrically. In Fe-EC, humic acid showed a non-linear response, with enhanced PFOS removal at low levels and suppression at ~0.8 mg L⁻¹ humic acid, with partial removal at higher levels, likely via floc bridging. Alternately, Fe-EC–PDS was largely insensitive to humic-acid variation. To resolve the fate of the “unaccounted” fraction of PFOS, we evaluated two different extraction methods. Acid digestion of wet flocs recovered 28% PFOS (pre-digestion spike) and 102% PFOA (post-digestion spike) validating Fe-PFOS complex formation. Oven-drying-based extraction of flocs yielded only 0.05% PFOS (pre-dehydration spike) and 0.13% PFOA (post-dehydration spike), demonstrating irreversible immobilization of PFOS within dehydrated Fe–O–Fe networks and Fe–O–S coordination sites. Collectively, these results (i) confirm Fe-EC and Fe-EC-PMS/PDS as a non-destructive immobilization process, (ii) provide extraction-based evidence for floc-phase entrapment, and (iii) deliver design guidance for electrolytes and natural organic matter. This mechanistic clarity enables predictive Fe-EC pretreatment to condition PFAS-impacted streams prior to downstream adsorption or membrane polishing. Keywords: Perfluorooctane sulfonic acid, Iron-Electrocoagulation, Persulfate Activation, Extraction validation, Floc Immobilization