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

PFAS are persistent organofluorine compounds with exceptionally strong C–F bonds, leading to environmental persistence, bioaccumulation, and health concerns. They are frequently detected in water and human biomonitoring, with PFOA having a human half-life of ~2.3 years and associated adverse health effects. Conventional drinking water treatments (coagulation, flocculation, sedimentation, filtration) are largely ineffective due to PFAS high solubility and low volatility. While adsorption (e.g., activated carbon, ion exchange) and membrane processes remove PFAS from water, they require regeneration or concentrate management. Photocatalytic degradation using metal oxides has emerged as a promising destruction pathway. This study aims to develop and evaluate a simple, low-cost hybrid photocatalyst—iron oxide immobilized on graphenic carbon (Fe/g-C)—to enhance adsorption, charge separation, and active sites, enabling efficient UV-driven degradation of PFOA under fluence-controlled conditions.

The paper reviews limitations of conventional PFAS treatments and highlights alternative technologies: adsorption using activated carbon, ion exchange resins, and modified clays; membrane filtration; and destructive processes including photocatalysis. Several metal oxides (ZnO, CeO2, Ga2O3, TiO2) under UV have been evaluated for PFAS decomposition. The ideal PFAS photocatalyst should combine high activity/selectivity, large surface area, stability, and cost-effectiveness. Graphenic carbons can enhance adsorption kinetics and provide high surface areas and mesoporosity, while iron oxides offer photoactivity and facilitate charge separation. The study situates the Fe/g-C hybrid against reported photocatalysts via a normalized degradation efficiency (NDE) metric that accounts for time, UV fluence rate, and catalyst dosage normalized to PFAS concentration, enabling fair comparison across studies.

Synthesis: Graphenic carbon (g-C) and Fe-doped g-C were synthesized from Chemi-ThermoMechanical pulp (CTMP). Fibers were milled (40 OD mesh), soaked in 20 mL FeCl3 solutions of varying concentrations to yield targeted Fe loadings (1.6–64 wt%), air-dried, re-milled (40 OD mesh), then pyrolyzed at 600 °C for 5 min in a muffle furnace (heating rate 40 °C min−1) under an oxidative atmosphere. The furnace was turned off at 600 °C to allow gradual cooling to room temperature. Characterization: SEM (Quattro ESEM, 10 kV, ~1200×) examined morphology; TGA (TGA 5500) under N2 from 25–850 °C at 20 °C min−1 assessed thermal stability; powder XRD (Rigaku MiniFlex 6G) identified crystalline phases; FTIR (Bruker Invenio, ATR, 4000–400 cm−1, 4 cm−1 resolution, 128 scans) probed functional groups; Raman (Renishaw inVia, 532 nm, low laser power) analyzed D and G bands with Lorentzian (D) and BWF (G) fits. N2 physisorption (Micromeritics 3Flex, 77 K) after degassing at 150 °C for 24 h provided BET surface area, pore volumes, t-plot analysis, and DFT pore size distributions. UV–Vis diffuse reflectance (Agilent Cary 5000, 250–2500 nm) with Spectralon reference and Kubelka–Munk/Tauc (indirect transitions) estimated optical bandgaps. XPS (Kratos AXIS Nova, Al Kα, charge neutralizer on; pass energies 160 eV survey, 20 eV HR; charge-corrected to C 1s at 284.8 eV) analyzed surface states and changes after reactions. Photocatalytic tests: A collimated-beam bench-scale photoreactor (254 nm UV) with fluence measurement was used. Typical experiments dispersed 50 mg catalyst in 50 mL of 1 mg L−1 PFOA at initial pH 3.5 (dosage 1 g L−1), stirred 30 min in the dark to reach adsorption–desorption equilibrium, preheated the UV lamp 30 min, then irradiated at room temperature (22 ± 2 °C). Sampling: 2 mL aliquots at set intervals. Lighting conditions compared UV alone, dark control, and simulated solar (AM1.5G). Catalyst dosage and initial PFOA concentration (0.1–5 mg L−1) effects were evaluated. Analytical methods: PFOA quantified by UHPLC/MS (Agilent 1200, Waters XTerra MS C18 100×2.1 mm, 3.5 μm, with guard column; 50 °C; 1 mL min−1; mobile phases: water + 20 mM ammonium acetate (A) and acetonitrile (B); gradient 50:50 to 10:90 0–5 min, back to 50:50 at 5–5.5 min, hold to 8 min). MS in negative ESI, MRM; drying gas 325 °C, 1 L min−1; nebulizer 344.7 kPa; capillary 4000 V; 10-point calibration 0.5 μg L−1 to 2 mg L−1. Post-photodegradation iron quantified by ICPMS (Agilent 7700x); calibration 0.1–100 ppb; samples diluted 1000× when out of range; certified reference materials TM25.3, TM26.3 used. XPS O 1s and F 1s analyzed before/after cycles to assess fluoride adsorption and surface functional group changes. Comparative metric: Normalized degradation efficiency (NDE) defined as NDE = time[h] × UV fluence rate [mW cm−2] × catalyst dosage [g L−1] / PFOA concentration [mg L−1] for comparison with literature datasets reporting all required parameters.

• The Fe/g-C hybrid outperformed literature-reported photocatalysts in normalized degradation efficiency (NDE), using readily available, low-cost materials and simple fabrication.
• Structural and textural properties: XRD showed increased crystallinity with Fe loading and phases including α-Fe2O3, γ-Fe2O3, Fe3O4, and α-FeOOH on graphenic carbon. Raman spectra exhibited D (1373 cm−1) and G (1589 cm−1) bands with ID/IG increasing linearly with Fe content, indicating higher structural order supportive of improved charge transfer. FTIR indicated aromatic C=C (~1584 cm−1), C=O (~1705 cm−1) decreasing with Fe, C–O (~1192 cm−1), O–H (3200–3500 cm−1), and Fe–O (~542 cm−1). BET surface area increased from ~56 m2 g−1 (g-C) to ~427 m2 g−1 (32 wt% Fe/g-C); pore diameter increased from 0.92 to 13.4 nm. Optical bandgaps were 1.28 eV (g-C) and 1.19 eV (32 wt% Fe/g-C).
• Photocatalytic performance (254 nm UV, fluence rate 1.42 ± 0.05 mW cm−2, temperature 22 ± 2 °C): UV alone showed negligible PFOA decay (ε254 ~3.5 ± 0.9 M−1 cm−1). Fe/g-C with 1.6, 3.2, 4.8, 6.4 wt% Fe achieved 3.7%, 5.8%, 7.2%, 13.4% removal in 6 h (30.7 J cm−2). Higher Fe loadings yielded substantial removal: 16 wt% Fe (66.4%) and 32 wt% Fe (89.7%) in 6 h. A 64 wt% Fe/g-C sample did not further enhance degradation. Approximately ≥85% removal was achieved within 3 h under UV, and ~90% in 6 h.
• Catalyst dosage and initial concentration effects: Even at 0.1 g L−1 catalyst, ~79% removal in 6 h was observed. Increasing initial PFOA concentration from 0.1 to 5 mg L−1 decreased 6 h removal from near-complete to ~86%.
• Light condition comparison: Removal was highest under simulated solar light (AM1.5G) compared to UV 254 nm and dark conditions; dark controls showed significant adsorption but lower total removal than under irradiation, supporting a dual adsorbent–photocatalyst role.
• Stability and recyclability: In five consecutive cycles (~30 h total), 32 wt% Fe/g-C removed >90% PFOA in cycles 1–4 and 88.5% in cycle 5 without chemical regeneration; SEM showed no morphological degradation.
• Fluoride mass balance and adsorption: Total fluoride recovery decreased to ~42.2% by 1.5 h and then stabilized, attributed to fluoride adsorption onto iron oxide sites. Control with NaF showed ~47% F adsorption on 32 wt% Fe/g-C; pure g-C showed negligible F adsorption. XPS after reaction showed an F 1s peak at 648.5 eV (adsorbed fluoride). Iron leaching after photodegradation was measured at ~69.95–70.21 ppm (ICPMS).
• Mechanistic insights: Two-step process—adsorption of PFOA on Fe/g-C followed by hole transfer-driven oxidation and defluorination. XPS O 1s deconvolution indicated reduction in oxygen-containing groups post-reaction; electrons from α-Fe2O3 migrate to g-C, with oxygen functionalities acting as electron acceptors to suppress recombination, facilitating efficient hole-driven oxidation on Fe/g-C.
• Practical significance: Consistent ≥85% degradation for 30 h at fluence rate 1.42 ± 0.05 mW cm−2 using a low-cost, scalable hybrid material fabricated via a simple pyrolysis of FeCl3-impregnated cellulose.

The study addresses the challenge of PFAS persistence and the limitations of traditional removal methods by demonstrating a hybrid photocatalyst that combines adsorption capacity with effective photo-induced degradation. The Fe/g-C architecture enhances contact between PFOA and reactive sites via high surface area and mesoporosity, while dispersed iron oxide nanoparticles provide photoactivity and promote charge separation. The increased structural order (higher ID/IG) and tailored surface chemistry (oxygen-containing functional groups and iron oxide phases) facilitate electron transfer from hematite to the carbon matrix, reducing recombination and enabling effective hole-driven oxidation of adsorbed PFOA. Performance scales with Fe loading up to an optimum (16–32 wt%) beyond which further Fe addition (64 wt%) does not improve activity, likely due to changes in dispersion or light absorption/shading. The photocatalyst demonstrates robust operation across catalyst dosages and PFOA concentrations relevant to natural waters and concentrated waste streams, with high removal under UV and improved performance under simulated solar irradiation. The comparative NDE analysis indicates that Fe/g-C achieves superior efficiency relative to literature systems while using simple, inexpensive components, supporting its potential for practical water treatment applications.

A simple, frugal synthesis of iron oxide/graphenic carbon hybrids from cellulose yields a heterogeneous photocatalyst that efficiently degrades PFOA. The 32 wt% Fe/g-C achieves ≥85% degradation within 3 h and ~90% within 6 h under low UV fluence rates, maintains ≥85% performance over 30 h, and is recyclable over at least five cycles without morphological degradation or chemical regeneration. Structural (Raman, XRD), textural (BET, porosity), optical (DRS/Tauc), and surface (XPS) analyses elucidate the relationship between Fe content, structural order, charge transfer, and photocatalytic performance, supporting a dual adsorbent–photocatalyst mechanism with adsorption followed by hole-driven oxidation. This work suggests that low-cost, scalable graphenic carbons doped with iron oxides can enable practical PFAS degradation at reduced energy input. Future research should explore optimization of Fe loading and dispersion, mitigation of fluoride adsorption and iron leaching, extension to diverse PFAS structures and real water matrices, and integration with solar-driven systems or continuous-flow reactors for scale-up.

• Fluoride mass balance was incomplete (total fluoride recovery ~42% after 1.5 h), attributed to adsorption of fluoride on iron oxide sites, complicating defluorination quantification and potentially requiring desorption steps for accurate mass balance and byproduct management.
• Detectable iron leaching (~70 ppm) occurred post-photodegradation; leaching control and catalyst stabilization require further optimization for practical deployment.
• Optimal performance depended on UV irradiation; while simulated solar improved removal, reliance on light sources may constrain some applications without solar integration.
• Excessive Fe loading (e.g., 64 wt%) did not improve performance, indicating a narrow optimal composition window and potential trade-offs with light absorption and active site accessibility.
• Experiments were conducted in controlled laboratory matrices (initial pH ~3.5) with PFOA as the model PFAS; broader PFAS classes, variable water chemistries, natural organic matter, and real wastewaters need validation.
• Bandgap and optical properties of the composites suggest limited visible absorption for some configurations; further tuning may be required to maximize solar utilization.