Per- and polyfluoroalkyl substances (PFAS) are synthetic chemicals resistant to degradation, posing significant environmental and health risks. Their persistence stems from the strong carbon-fluorine bonds. PFAS are widely used in various industrial and consumer applications, leading to their frequent detection in aquatic environments and human blood samples globally. The high water solubility and low vapor pressure of many PFAS render conventional water treatment methods ineffective. Current removal methods, such as adsorption (using activated carbon, ion exchange resins, and modified clays) and membrane filtration, often prove insufficient or costly. Advanced destruction techniques, including photocatalytic degradation, offer a promising alternative. This research explores a novel, cost-effective method utilizing an iron oxide/graphenic carbon (Fe/g-C) hybrid photocatalyst to address PFAS contamination in water. The approach leverages the photoactivity of iron oxide and the high surface area of graphenic carbon to enhance adsorption kinetics and provide numerous active sites for catalysis.

Existing PFAS degradation methods include adsorption using various sorbents, membrane filtration, and advanced destruction techniques. While activated carbon demonstrates high PFAS adsorption capacity, regeneration or disposal is needed. Advanced oxidation processes show promise but often require complex procedures. Several metal oxides (ZnO, CeO₂, Ga₂O₃, TiO₂) have been investigated for PFAS decomposition under UV irradiation. An ideal photocatalyst needs high activity, selectivity, broad surface area, excellent stability, and cost-effectiveness. Immobilizing photoactive transition metal oxides on mesoporous carbon enhances adsorption kinetics and provides a large surface area for the reaction.

The Fe/g-C hybrid photocatalysts were synthesized by pyrolyzing cellulose impregnated with varying concentrations of iron chloride (FeCl₃). The synthesis involved milling cellulose into fine powder, soaking it in FeCl₃ solutions, drying, and subsequent pyrolysis at 600 °C for 5 minutes under an oxidative atmosphere. The resulting materials were characterized using various techniques including Scanning Electron Microscopy (SEM), Powder X-ray Diffraction (PXRD), Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and N₂ physisorption analysis. The photocatalytic activity was evaluated in a custom-designed photoreactor using UV light (λ = 254 nm) and simulated solar light. The decomposition of PFOA (at various concentrations) was monitored using ultra-high-performance liquid chromatography/mass spectrometry (UHPLC/MS). The recyclability and stability of the photocatalyst were tested over five consecutive cycles. Post-photodegradation iron leaching was determined using Inductively Coupled Plasma Mass Spectrometry (ICPMS). High-resolution XPS analysis investigated the adsorption process and PFOA photodegradation.

The Fe/g-C hybrid photocatalyst demonstrated superior performance compared to other photocatalysts reported in the literature. With 32 wt% Fe/g-C, >85% PFOA removal was achieved within 3 hours under UV irradiation (fluence rate of 1.42 mW cm⁻²) at an initial PFOA concentration of 1 ppm. The normalized degradation efficiency (NDE) calculation (Eq. 1 in the paper) further confirmed its superior performance. SEM images revealed a heterogeneous mixture of particles with iron oxide crystallites well-dispersed on the carbon substrate. PXRD, FTIR, and Raman analyses confirmed the presence of iron oxides (α-Fe₂O₃, γ-Fe₂O₃, Fe₃O₄, α-FeOOH) and the graphenic carbon structure. The BET surface area of 32 wt% Fe/g-C was significantly higher (~427 m² g⁻¹) than that of pure g-C (~56 m² g⁻¹). The optical band gaps of the composites were in the range of 1.19-1.28 eV. Increasing the photocatalyst dosage improved PFOA removal, with high removal efficiency even at low dosages. Increasing the initial PFOA concentration decreased removal efficiency; however, substantial removal (~85%) was still observed at concentrations representative of concentrated waste streams. The catalyst demonstrated dual functionality as an adsorbent and photocatalyst. The catalyst maintained >90% PFOA removal efficiency over five consecutive cycles (~30 hours), indicating high recyclability and stability. XPS analysis revealed fluoride adsorption on the photocatalyst surface after the reaction and a reduction in oxygen-related functional groups on the g-C support after photocatalysis. Total fluoride recovery was lower than expected, suggesting fluoride adsorption onto the catalyst's surface. Iron leaching was minimal (69.948 to 70.210 ppm).

The high efficiency of the Fe/g-C hybrid photocatalyst for PFOA degradation can be attributed to the synergistic effect of iron oxide and graphenic carbon. Iron oxide facilitates charge separation and drives the photocatalytic process, while the graphenic carbon provides a large surface area for adsorption and efficient charge transfer, minimizing electron-hole pair recombination. The observed dual functionality (adsorption and photocatalysis) enhances the overall degradation efficiency. The excellent stability and recyclability of the catalyst make it a promising candidate for practical PFAS remediation applications. The lower-than-expected fluoride recovery suggests that some fluoride ions might be adsorbed onto the catalyst surface, mitigating the potential release of this harmful byproduct. The minimal iron leaching confirms the stability of the catalyst during the extended irradiation.

This study demonstrates a simple, cost-effective method for synthesizing a highly efficient and stable Fe/g-C hybrid photocatalyst for PFAS degradation. The catalyst exhibits superior performance compared to other reported methods, achieving >85% PFOA removal within 3 hours under UV irradiation. Its recyclability and stability make it a promising candidate for large-scale PFAS remediation. Future research could focus on exploring other sustainable graphenic carbon sources and investigating the catalyst's effectiveness for other PFAS compounds and under different light sources.

The study primarily focused on PFOA degradation. Further research is needed to assess the effectiveness of the Fe/g-C hybrid photocatalyst for other PFAS compounds with varying chain lengths and functional groups. The experiments were conducted under controlled laboratory conditions. The performance of the catalyst in real-world scenarios with complex water matrices needs to be evaluated. A detailed mechanistic study might be beneficial to further understand the PFOA degradation pathways and the roles of different components within the Fe/g-C hybrid.