Strategies to advance heterogeneous Fenton processes for water treatment

Advanced oxidation processes (AOPs) have gained widespread attention due to their exceptional efficiency in removing a diverse range of pollutants from wastewater. The homogeneous Fenton process, a first-generation AOP, involves the reaction of Fe²⁺ with H₂O₂ under acidic conditions to generate hydroxyl radicals (•OH), which are highly reactive in pollutant oxidation. Despite its efficacy, homogeneous Fenton faces challenges such as the continuous addition of Fe²⁺, a narrow pH working range and iron sludge generation. Heterogeneous Fenton processes have been proposed to address these issues by utilizing solid catalysts, e.g., iron-based materials, to generate •OH on their surface. However, they still have certain limitations that need to be addressed for broader wastewater treatment. One of these challenges is their relatively low efficiency in pollutant removal, which is attributed to the limited number of active sites on the catalyst surface and the accessibility of mass transfer between the liquid phase and the catalyst surface. Second, they suffer from low selectivity, leading to the insufficient oxidation of target pollutants. The third challenge lies in their adaptability to wild conditions, such as high pH levels and COD. Lastly, the application potential of these processes is a significant concern, as H₂O₂ and catalysts are expensive. Therefore, this thesis mainly focuses on these limitations and provides innovative solutions.

To improve treatment efficiency, an FeCN₅ catalyst combining single-atom and cluster sites was simply prepared by controlling the ratio of precursors. This novel catalyst exhibited a record oxidation rate of 59.43 mg/L min⁻¹ for methylene blue, but it also showed broad oxidation capability in terms of removing multiple organic pollutants via direct, ultrafast H₂O₂ activation. This excellent performance was mainly attributed to i) the co-presence of single atoms and clusters, which led to the enhanced exposure of active sites, ii) the FeCN5 catalyst creating an acidic microenvironment, further facilitating the generation of •OH, and iii) the leaching of Fe ions from the catalyst, which contributed to assisting the homogenous Fenton process.

A catalyst surface engineering strategy was employed to increase the selectivity issue for Fenton catalysts. The catalysts acquired negative surface charges by using carbon nanotube (CNT)-supported Fe₂O₃ with varying Fe₂O₃ contents, prepared through one-pot pyrolysis. This characteristic enabled them to selectively remove positively charged pollutants, with varying efficiencies. Mechanisms demonstrated that selective removal was achieved by i) the surface enrichment of target pollutants via electrostatic interactions, ii) the synergy of homogenous and heterogeneous Fenton processes in different phases and iii) the generation of •O₂- and recycling of Fe³⁺/Fe²⁺. FeOCl was ingeniously engineered in a separate work through a two-step strategy involving polyaniline intercalation and dodecyl group modification. The engineered catalyst displayed increased layer distance and a hydrophobic surface, which in turn increased its activity and selectivity in relation to removing hydrophobic pollutants

To enhance the environmental adaptability and sustainability of the catalyst, various materials were selected for immobilizing the above-mentioned catalysts. For instance, the FeCN₅ catalyst was modified on the carbon felt surface and integrated into a plastic column to form a flow-through Fenton filter. This filter achieved nearly 100% pollutant degradation and H₂O₂ utilization over 80 h in actual wastewater. Crucially, it leached low Fe ions (< 0.2 mg/L) throughout the entire process. In addition, the engineered FeOCl was encapsulated within commercially available corundum balls to develop “millimeter-scale reactors”. This strategy reduced the exposure of the catalyst to external environments and maintained a catalyst efficiency of 86.02% after 10 cycles, which is significantly higher than other material-supported catalysts. Importantly, it exhibited negligible physical changes, even after prolonged use.

A novel electrochemical platform was developed for wastewater treatment applications. The platform’s construction involved a vertical bidirectional gas diffusion electrochemical system (VB-GDE), which combined anodic and cathodic reactions to create an acidic environment and accumulate H₂O₂ up to 6.97 mM within 1 h. Moreover, the catalyst was incorporated into the VB-GED to develop a wastewater treatment platform, which integrated the catalyst-initiated Fenton reaction with direct anodic oxidation, thereby ensuring highly efficient pollutant removal. A power-on-off mode was applied to the platform to achieve a sustainable goal. This mode allowed for precise control over the Fenton reaction, thus minimizing chemical residue and conserving energy during treatment.