A Zwitterionic Hydrogel-Based Heterogeneous Fenton Catalyst for Efficient Degradation of Persistent Organic Pollutants

Persistent organic pollutants (POPs) are industrial, pharmaceutical, or commercial byproducts that are highly resistant to degradation and decomposition in the environment, while causing significant health and environmental damage even in small concentrations. Significant existing research aims at destroying the POPs in an environmentally safe and cost-effective manner, but challenges remain in destroying POPs at environmental concentrations in scalable and sustainable processes. Many current approaches to degrading POPs require extreme reagents and conditions. One promising candidate, however, for the removal of POPs is the photo-Fenton reaction, where iron(II) ions catalyze the decomposition of hydrogen peroxide, in the presence of UV light, into highly reactive hydroxyl radicals that readily attack organic compounds, including POPs. Fenton oxidation is promising because iron(II) and hydrogen peroxide are low-cost, environmentally-safe materials, and because the process can target a broad range of POPs.<br/><br/>However, the photo-Fenton reaction requires the presence of water-soluble iron(II) ions that are difficult to reuse. As such, methods that immobilize the iron as a catalyst can be constantly reused, while also eliminating the need for further treatment steps to remove iron from water before it can be used. Unfortunately, existing solutions require challenging reaction conditions; due to the higher efficiency of the Fenton reaction at lower pHs, many catalysts require a low pH (usually around 3) to regenerate iron(II) ions, which is unrealistic for large-scale wastewater treatment and can damage the catalyst itself. Furthermore, due to the non-selectivity of the Fenton reaction, catalysts using common polymers such as polyethylene or polyvinyl alcohol to encapsulate iron (II) or iron oxide (in the form of nanoparticles) often degrade easily due to the susceptibility of the polymer to the photo-Fenton reaction, also releasing nanoparticles into the environment. Other attempts at supported catalysts reduce the surface area and limit kinetics.<br/><br/>To address these issues, we developed an iron(II)-incorporated, zwitterion-based hydrogel catalyst. By complexing the iron(II) ions within a zwitterionic hydrogel, we allow for the photo-Fenton reaction to run efficiently at neutral and even slightly alkaline pHs. Furthermore, due to the high oxidation state of the zwitterions and the saturated backbones, the hydrogel is highly resistant to attack by hydroxyl radicals. A zwitterionic backbone helps us make highly porous materials that bind individual, complexed ions, allowing us to maximize both the effective functional area and rate of pollutant transport within the catalyst. POPs quickly and rapidly diffuse into the catalyst, where they rapidly decompose. Experiments at environmentally relevant concentrations showed that our catalyst was able to degrade 85% of a solution of ethinyl estradiol (a xenoestrogen and common POP) within 1 hour, with near 100% degradation within 24 hours. Furthermore, we observed near 100% degradation of dichlorophenol (a common industrial byproduct) and around 70% degradation of perfluorooctanoic acid (a common perfluorinated pollutant). These results show significant promise for our hydrogel as an efficient and environmentally-safe strategy for rapid water purification of POPs.