Hybrid graphenic and iron oxide photocatalysts for the decomposition of synthetic chemicals

Per- and polyfluoroalkyl substances (PFAS) are persistent synthetic chemicals posing significant environmental and health risks due to their resistance to degradation. Current removal methods are often complex and expensive. The strong carbon-fluorine bonds in PFAS contribute to their persistence and bioaccumulation. PFAS are widely used in various industrial and consumer products, leading to their frequent detection in aquatic environments and human blood samples, exceeding safe levels in many regions. Conventional water treatment methods are ineffective against PFAS due to their high water solubility and low vapor pressure. Alternative remediation technologies, such as adsorption and advanced destruction techniques, are necessary. Photocatalytic degradation, using light energy to generate reactive species, shows promise. This research explores a novel, cost-effective method using an iron oxide/graphenic carbon (Fe/g-C) hybrid photocatalyst to overcome limitations of existing PFAS remediation approaches.

The literature review highlights the challenges posed by PFAS's resistance to conventional water treatment. It discusses existing remediation strategies including adsorption using activated carbon, ion exchange resins, and modified clays, noting limitations like regeneration or disposal needs. Advanced destruction techniques are explored, with photocatalytic degradation presented as a promising alternative. The review mentions research on various metal oxides (ZnO, CeO2, Ga2O3, TiO2) for PFAS decomposition under UV irradiation. The use of iron oxide's photoactivity and complexing ability in combination with mesoporous carbon to enhance adsorption kinetics and provide a large surface area for the reaction is highlighted as a potentially effective strategy.

The Fe/g-C hybrid photocatalysts were synthesized by pyrolyzing cellulose impregnated with varying concentrations of iron chloride (FeCl3). The synthesis procedure involved milling CTMP pulp, soaking it in FeCl3 solutions, drying, and then pyrolyzing at 600 °C for 5 minutes under an oxidative atmosphere. The resulting materials were characterized using several techniques: * **Scanning Electron Microscopy (SEM):** To examine morphology and microstructure. * **Thermogravimetric Analysis (TGA):** To assess thermal stability. * **Powder X-ray Diffraction (PXRD):** To determine crystalline structure and phase composition. * **Fourier Transform Infrared (FTIR) Spectroscopy:** To analyze chemical structure. * **Raman Spectroscopy:** To investigate structural changes in the graphenic carbon structure. * **N2 Physisorption:** To determine textural properties (surface area, pore volume, pore size distribution). * **UV-Vis Diffuse Reflectance Spectroscopy:** To evaluate optical band gap. * **X-ray Photoelectron Spectroscopy (XPS):** To investigate the surface electronic structure. Photocatalytic activity was evaluated in a custom-designed photoreactor using a UV lamp (λ = 254 nm). The decomposition of PFOA was monitored using UHPLC/MS. Experiments were conducted with varying iron content in the Fe/g-C, initial PFOA concentrations, and catalyst dosages. The recyclability and stability of the photocatalyst were assessed through multiple cycles of PFOA decomposition. Post-photodegradation iron leaching was determined using ICPMS. The total fluoride recovery was determined and the adsorption of fluoride on the photocatalyst surface was studied through XPS analysis.

A comparative analysis showed the Fe/g-C hybrid outperformed other photocatalysts reported in the literature in terms of normalized degradation efficiency (NDE). The optimal Fe/g-C hybrid (32 wt% Fe) achieved >85% PFOA removal within 3 hours under UV irradiation (fluence rate of 1.42 mW cm⁻²) at an initial PFOA concentration of 1 ppm. SEM images revealed the heterogeneous distribution of iron oxide crystallites on the graphenic carbon surface. PXRD, FTIR, and Raman spectroscopy confirmed the presence of α-Fe2O3, γ-Fe2O3, Fe3O4, and α-FeOOH phases along with graphitic and amorphous carbon. The BET surface area increased significantly with iron doping, enhancing photocatalytic activity. The optical band gap of the Fe/g-C hybrids was smaller than that of graphene or Fe2O3. Increasing the catalyst dosage further enhanced PFOA removal. Higher initial PFOA concentrations led to slightly lower removal efficiencies. The catalyst demonstrated excellent recyclability and stability over five consecutive cycles (>90% PFOA removal for the first four cycles, and 88.5% in the fifth). XPS analysis indicated fluoride adsorption onto the catalyst surface, and that oxygen-containing functional groups on the g-C support act as electron acceptors which decelerate the recombination of charges, improving photocatalytic efficiency. The leached iron concentration was low after the photodegradation.

The high efficiency of the Fe/g-C hybrid photocatalyst is attributed to a synergistic effect between the iron oxide's photoactivity and the graphenic carbon's high surface area and adsorption capacity. Iron oxide facilitates charge separation and generates reactive species, while the graphenic carbon enhances adsorption of PFOA molecules, promoting efficient degradation. The reduced optical bandgap of the hybrid compared to individual components also contributes to improved photocatalytic activity. The catalyst's stability and recyclability suggest its potential for practical application in PFAS remediation. The relatively low fluoride recovery indicates fluoride adsorption onto the catalyst, which requires further investigation. The observed results demonstrate a significant improvement over existing methods by achieving high degradation rates with a simple, cost-effective synthesis and low UV fluence.

This study successfully demonstrates a simple, cost-effective method for synthesizing a highly efficient and stable Fe/g-C hybrid photocatalyst for PFAS degradation. The catalyst achieved over 85% PFOA removal within 3 hours under UV irradiation and maintained high activity over multiple cycles. Future research could focus on exploring other sustainable sources of graphenic carbon, optimizing the synthesis parameters, and evaluating the catalyst's performance in real-world contaminated water samples.

The study primarily focused on PFOA degradation under controlled laboratory conditions. Further research is needed to evaluate the catalyst's performance with other types of PFAS and in more complex water matrices. The low recovery of fluoride requires further research to establish the fate of fluorine. The long-term stability and effects of potential fouling in continuous operation require more extensive investigation. The scale-up for large-scale applications needs further evaluation.