Sarah Sühnholz

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 KD value of 1.8 × 104 L/kg at a cfree 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 FeIV=O2+ species, generated by the oxidation of Fe(OH)(H2O)52+-PFOA complex by SO4•−. The FeIV=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 cfree 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.

Increasing environmental awareness and still countless cases of polluted environmental compartments have led to a wide range of publications on chemical remediation methods in recent years. Many of them are devoted to advanced oxidation processes (AOPs) in water, with increasing attention for the remediation of persistent pollutants in surface-, ground- and waste waters with sulfate radicals. Processes based on activation of the oxidants peroxydisulfate (PS) and peroxymonosulfate (PMS) are becoming more and more popular especially for in-situ remediation strategies for aquifers contaminated with perfluorinated alkyl substances (PFAS).In contrast to hydroxyl radicals, sulfate radicals are able to attack the carboxylate group in many PFAS. Our main focus of attention lies on the heterogeneous activation of PS for sulfate-radical generation in order to exploit pollutant enrichment at active surfaces. In the present study, the activation of PS with various iron minerals was investigated. From the pool of iron minerals tested, iron(II)-sulfide (FeS) proved to be a powerful activator for PS. A more detailed investigation of the FeS/PS system (e.g. pH dependence of the reaction, long-term performance of FeS as activator for PS and determination of radical yield and activation energy) led to the hypothesis of an activation mechanism via homolytic bond cleavage and to the assumption that FeS acts as a true catalyst with a considerable lifetime and not, as would be expected, as a reagent.Based on the experiences made, the FeS/PS system was optimized and specifically designed for degradation of PFOA (perfluorooctanoic acid), one of the most prominent PFAS representatives being in the focus of regulatory attention. Due to surface-mediated processes, the target reaction, PFOA degradation, can be carried out even in real groundwater samples, even additionally doped with typical water constituents, such as humic acid. These results are remarkable, as it is known that not only dissolved iron, but also chloride and dissolved organic matter (DOM) effectively quench free sulfate radicals. The presentation will inform about reaction products, yields and pathways under various reaction conditions. By mechanistic insight into the activation of PS via heterogeneous activation, this work breaks new ground for novel remediation approaches using PS for in-situ and ex-situ water treatment.Based on the promising lab results, a field test of the FeS/PS system, as a part of the Intraplex® technology, was conducted at a contaminated site in western Germany. The main contaminants were the perfluorinated carboxylic acids with a total concentration of 0.6 µg/L. Using a combination of permanent injection points and direct push injections, an injection transects was carried out to apply the material across the main downstream direction of the PFAS plume. The effects of the pilot test were studied by an intensive monitoring program which determined the concentration of pollutants, transformation products and hydrogeochemical parameters. Preliminary results will be presented.