Concerns linked to highly dispersed iron anchored within graphitic carbon nitride, is it a truly promising material to drive heterogeneous photo-Fenton treatments?

Abstract

The precipitation of iron at pH > 4 is one of the main drawbacks of any Fenton-based process. Among the engineered solutions, the incorporation of iron within the wide cavities of graphitic carbon nitride (g-CN) has recently gained momentum. However, most works employing Fe-g-CN materials usually employ high H₂O₂ concentrations (>25 mM) to observe considerable pollutant abatements (without or with UV–vis light irradiation, i.e., by heterogeneous dark- or photo-Fenton processes, respectively). To gain further insights into this issue, in this work, Fe-g-CN, with different amounts of iron, were synthesised by thermal polycondensation of melamine and FeCl₃·6H₂O as precursors and compared its performance with the g-CN alone. Under UV-A light, a content of 0.2% w/w of iron in the g-CN was optimal to improve the oxidative performances of target pollutants (phenol and sulfamethoxazole 100 μM, respectively), higher Fe-loadings decreased the photocatalytic performances with respect to g-CN. Interestingly, this trend was inversed when adding H₂O₂ 1 mM, being the pollutant removal by g-CN faster than that by Fe-g-CN (for phenol, k_obs = 8.02 × 10⁻² min⁻¹ and 2.83 × 10⁻² min⁻¹, respectively), opposed to expectations. Furthermore, HO^•, HO₂^• or ¹O₂ were barely detected by Electron Paramagnetic Resonance, indicating that the reactive species should oxidise the g-CN rather than react with the spin traps. Finally, although g-CN oxidation was not observed by typical characterisation techniques (such as FT-IR/ATR), we have observed 6 times more nitrates formation by illuminated Fe-g-CN than g-CN, indicating that iron enhances the self-oxidation of illuminated carbon nitrides. Our results demonstrate that iron incorporation in g-CN might be not as convenient as usually stated in the literature, as the stability of the photocatalyst is drastically reduced, releasing nitrates and possibly decreasing the material's lifetime.