Efficient Oxidation of Aqueous Persistent Organic Pollutants using Zwitterionic Hydrogel-Bound Iron(II) Ions as Heterogenous Fenton Catalysts

Persistent organic pollutants, such as xenoestrogens and PFAS, present a global challenge requiring immediate action. The Fenton oxidation reaction is an advanced oxidation process catalyzed by dissolved Fe(II) ions and their complexes, which converts hydrogen peroxide to hydroxyl radicals to degrade organic contaminants. Though the Fenton reaction is extremely promising for scalable water treatment, there are several limitations to its practical use, such as the difficulty in transporting and storing hydrogen peroxide. Further, though Fe(II) ions are called catalysts in Fenton processes, they are continuously oxidized to Fe(III) ions and are difficult to reuse or recover from water. Attempts to create a practical Fenton process have used electrochemical cells to produce hydrogen peroxide <i>in situ</i>, while replacing dissolved Fe(II) ions with heterogenous catalysts. Such catalysts typically encapsulate iron or iron oxide nanoparticles in polymers or support them on traditional adsorbents but have so far been limited by slow reaction rates due to reduced surface area, the need to adjust pH with acids, unintended substrate/polymer degradation by the Fenton reaction, and the release or slow dissolution of nanoparticles. These catalysts are also incompatible with other improvements to the traditional Fenton reaction, such as UV light to accelerate the production of hydroxyl ions.<br/><br/>A practical Fenton process to eliminate POPs will require the development of (1) a heterogenous catalyst that retains iron while (2) eliminating the need for acid addition, (3) is highly porous and has a large effective surface area to preserve efficacy, (4) is transparent to UV radiation and (5) resistant to degradation by UV light, hydrogen peroxide, and the iron itself, (6) preferably does not use nanoparticles which may pose a threat to the environment if they escape the catalyst over long periods of time, and (7) can be regenerated and reused in a facile manner. Here, we introduce an innovative heterogeneous Fenton catalyst based on a zwitterionic hydrogel material that effectively addresses these challenges. Containing individually complexed iron ions in a highly porous scaffold, the hydrogel catalyst has a large effective surface area and exhibits kinetics comparable to homogeneous Fenton degradation. The complexed ions can initiate Fenton degradation at neutral, and even alkaline pH, eliminating the need for acid additions. At the same time, the zwitterionic hydrogel scaffold is specifically selected to be resistant to Fenton oxidation and strongly bind to the iron ions, enabling repeatable long-term use. Using our catalytic hydrogels, we showcase the rapid and complete degradation of three structurally disparate contaminants of major concern, ethinyl estradiol (a xenoestrogen), 2,4-dichlorophenol (a model pesticide and chlorinated aromatic) and perfluorooctanoic acid (PFOA, a model for PFAS), over multiple cycles of use at environmentally relevant concentrations. Our zwitterionic hydrogel-based Fenton catalyst offers a promising solution for the efficient and scalable degradation of persistent organic pollutants in electrochemical water treatment processes and beyond.