Parameters affecting LED photoreactor efficiency in a heterogeneous photo-Fenton process using iron mining residue as catalyst

LED-irradiated photo-Fenton process on pollutant removal: outcomes, trends, and limitations

This manuscript critically reviews the state of the art on the application of photo-Fenton processes irradiated by light-emitting diode arrays (LED) with a focus on the removal of contaminants of emerging concern (CEC) from aqueous matrices. LEDs are clean, low-cost radiation sources with longer lifespan compared to mercury lamps. This study covers the influence of LED sources, wavelengths, and dose upon CEC removal, and the potential for disinfection, abatement of antibiotic-resistant bacteria (ARB), and genes (ARG). The bibliographic search was performed in Scopus database using keyword combinations and resulted in a portfolio containing 52 relevant articles published between 2010-2023. According to reviewed papers, LED photoreactor design has evolved in the past decade aiming to improve CEC degradation in aqueous matrices while reducing construction and operation costs, and energy consumption. Among several reactors (annular, fluidized bed, parallel plate, wireless, pathway systems, and microreactor) surveyed for their performance and scalability, LED chips and strips are particularly suitable for application due to their wide emission angle (≈120°) and small size (mm2), which allow for, respectively, efficient illumination coverage and flexible arrangement and design. LED microreactors are very efficient in the degradation of contaminants and scalable with reduced area requirements. Although most studies were performed in synthetic solutions and at laboratory scale, the externally LED irradiated cylindrical reactor was successful for application in full-scale municipal water treatment plants. Future studies should focus on evaluating CEC removal in wastewater using scalable devices for continuous operation of solar photo-Fenton at night.

Modification of a Brazilian natural clay and catalytic activity in heterogeneous photo-Fenton process

The catalytic activity of a Brazilian natural clay modified with the immobilization of iron oxide was applied in the heterogeneous Fenton process for the degradation of the antibiotic sulfathiazole (STZ). The clay without any treatment indicated a lamellar type material with mesoporous distribution that presents a heterogeneous mixture of phases (type 1:1 and 2:1 structures), with a predominance of quartz, montmorillonite, gibbsite and kaolinite, and with SiO₂, Al₂O₃, Fe₂O₃, K₂O, TiO₂, MgO as major oxides. Its high absorption in the UV-Vis ranges with a bandgap energy of 1.9 eV was attributed to the presence of hematite. It was observed that the effects of the addition of starch before heat treatment, and impregnation with iron, modified the clay surface. F rom the X-ray photoelectron spectroscopy analysis it was concluded that a structural reorganization is related to the conversion of the various iron oxide phases into hematite, as well as promoting an increase in Fe²⁺/ Fe³⁺ redox reactions allowing rapid degradation of STZ. The catalyst impregnated with iron and treated at 600 °C showed to be an economical and versatile catalyst with high catalytic efficiency (>97% STZ degradation after 60 min), with small differences according to the type of LED device used. Furthermore, it presented high stability and reusability reaching 93% degradation of STZ after four cycles of reuse with low consumption of H₂O₂.

A new approach on synergistic effect and chemical stability of graphene oxide-magnetic nanocomposite in the heterogeneous Fenton degradation of caffeine

Two compositions of graphene oxide-magnetite nanocomposites were studied as catalysts in the heterogeneous Fenton process. Transmission electron microscopy and X-ray diffraction revealed that the graphene oxide sheets were covered with nanoparticles of magnetite, with an average crystallite size of 7 nm. Infrared spectroscopy analysis indicated that the phases interacted through covalent Fe-O-C bonds. The composites presented significantly improved catalytic activity, compared to pure magnetite, with a synergistic effect of up to a factor of 17.1 for the Fenton degradation of caffeine, achieving total removal after 90 min. This synergistic effect was a consequence of the interaction between the phases, resulting in improved mass transfer of caffeine to the catalyst surface, adsorption and efficient degradation, with enhanced HO^• generation. The surface reaction constant increased by up to three orders of magnitude, demonstrating the important role of graphene oxide in the degradation kinetics of the heterogeneous Fenton process. The surface-bonded hydroxyl radicals were responsible for caffeine degradation, achieving 9.4 μmol L^-1. After five degradation cycles, a loss of Fe-O-C bonds and increase in oxygenated groups were associated with a small decrease of caffeine removal efficiency, from 98 to 82%, without significant iron leaching, in the dark, and with low consumption of hydrogen peroxide.

Nontronite mineral clay NAu-2 as support for hematite applied as catalyst for heterogeneous photo-Fenton processes

This study describes the characterization of Nontronite, a clay mineral with high content of structural iron, before and after iron incorporation and 600 °C heat treatment. The Nontronite was classified as a mesoporous material, with high absorption in the UV-Vis range and band gap energy of 1.9 eV, indicative of the presence of superficial hematite, also verified in XRD analysis. The heat treatment promoted a structure rearrangement and the conversion of other iron phases to hematite, allowing the formation of surface irregular sites on Nontronite and facilitating the access for the decomposition of H₂O₂ into HO. Its catalytic activity in heterogeneous photo-Fenton process was evaluated during the degradation of the antibiotic sulfathiazole (STZ) and showed high activity achieving undetectable levels of STZ after 20 min under UV-LED irradiation and solar irradiation, and showing no iron leaching under controlled pH = 3. The degradation intermediates identified indicated hydroxylation as the main degradation route.

Fenton-like degradation of sulfathiazole using copper-modified MgFe-CO3 layered double hydroxide

The catalytic activity of layered double hydroxides, with and without insertion of copper, was evaluated in a heterogeneous Fenton process for degradation of the antibiotic sulfathiazole (STZ). The characterizations with different techniques revealed lamellar structures formed by stacking of layers containing magnesium, iron, and copper cations. The insertion of copper in the lamellar structure increased the specific area of the material and the degradation kinetics, achieving complete STZ removal after 90 min. X-ray photoelectron spectroscopy analysis showed the presence of Cu(II) and Cu(I) surface sites, which contributed to the generation of hydroxyl and hydroperoxyl/superoxide radicals. It also indicated an increase of Cu(I) content after use. For both materials, but specially for LDH without copper, addition of tert-butyl alcohol and p-benzoquinone hindered STZ degradation, indicating the importance of hydroxyl and hydroperoxyl/superoxide radicals in the degradation process, respectively. These results demonstrated the potential of copper-modified MgFe-CO₃ as a catalyst for the degradation of emerging contaminants, offering the benefits of easy preparation and high efficiency in the Fenton process.