New insight into the microstructure of natural calcined laterites and their performance as heterogeneous Fenton catalyst for methylene blue degradation

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.

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.

Factors Affecting Alkali Activation of Laterite Acid Leaching Residues

In this experimental study, the alkali activation of acid leaching residues using a mixture of sodium hydroxide (NaOH) and alkaline sodium silicate solution (Na2SiO3) as activators is investigated. The residues were also calcined at 800 and 1000 °C for 2 h or mixed with metakaolin (MK) in order to increase their reactivity. The effect of several parameters, namely the H2O/Na2O and SiO2/Na2O ratios present in the activating solution, the pre–curing time (4–24 h), the curing temperature (40–80 °C), the curing time (24 or 48 h), and the ageing period (7–28 days) on the properties of the produced alkali activated materials (AAMs), including compressive strength, porosity, water absorption, and density, was explored. Analytical techniques, namely X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and elemental mapping analysis were used for the identification of the morphology and structure of the final products. The experimental results show that the laterite acid leaching residues cannot be alkali activated in an unaltered state, and the compressive strength of the produced AAMs barely reaches 1.4 MPa, while the mixing of the residues with 10 wt% metakaolin results in noticeably higher compressive strength (41 MPa). Moreover, the calcination of residues at 800 and 1000 °C has practically no beneficial effect on alkali activation. Alkali activated materials produced under the optimum synthesis conditions were subjected to high temperature firing for 2 h and immersed in distilled water or acidic solution (1 mol L−1 HCl) for 7 and 30 days in order to assess their structural integrity under different environmental conditions. This study explores the potential of alkali activation of laterite leaching residues amended with the addition of metakaolin for the production of AAMS that can be used as binders or in several construction applications in order to enable their valorization and also improve the environmental sustainability of the metallurgical sector.

Preparation of Manganese Dioxide Nanoparticles on Laterite for Methylene Blue Degradation

The laterite-coating manganese dioxide nanoparticle material (M2) prepared by the immersion method was used for the efficient removal of methylene blue (MB) from aqueous solution. The adsorption and heterogeneous Fenton catalytic oxidation experiments of M2 were investigated by changing the effective factors such as time, pH, amount of M2, and concentration of MB. The adsorption data of M2 showed good fitting with the Langmuir isotherm, suggesting that the adsorption of MB on the surface of M2 is a heterogeneous and physical adsorption process. Degradation of MB was also carried out to evaluate the heterogeneous Fenton catalytic oxidation characterization of a new catalytic oxidation material (M2). The results show that the M2 material has both adsorption and heterogeneous Fenton catalytic oxidation. However, the heterogeneous Fenton catalytic oxidation of the M2 material is the main performance. Hence, our groups have investigated the ability of the catalytic column treatment with high efficiency of 98–100% and the degradation efficiency after the sample running through the column almost does not change much. This proves that heterogeneous Fenton catalytic activity of the catalytic column is completely unaffected and reused many times after oxidizing MB. Specifically, even if the M2 material is reused for five times, the degradation efficiency still reaches 98.86%.