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Highly effective Fe3O4/Mn3O4/reduced graphene oxide (rGO) hybrids were synthesized as a heterogeneous catalyst for the degradation of organic dyes in aqueous solution using sulfate radical-based advanced oxidation processes. The physicochemical properties of the composite were characterized by several techniques, such as X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectra (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET) theory. The effects of different parameters on the catalytic activity of Fe3O4/Mn3O4/rGO, including initial Methylene Blue (MB) concentration, peroxymonosulfate (PMS) concentration, catalyst dosage, pH value, and temperature were assessed. Typically, 98.8% removal of 50 mg L−1 of MB and 68.3% reduction of TOC could be achieved in 30 min under the following conditions: temperature 25 °C, 100 mg L−1 of catalyst, and PMS dosage of 0.3 g L−1, showing a significant enhancement of the catalytic activity of the catalyst in the degradation of organic pollutants in aqueous solution compared with Fe3O4/rGO and Mn3O4/rGO. The catalyst exhibited high stability and good reusability according to three successive repeated reactions. Based on the radical experiments, the catalytic activity of Fe3O4/Mn3O4/rGO hybrids for degradation of MB is closely related with the amount of the sulfate and hydroxyl radicals generated from PMS. The excellent catalytic performance of the Fe3O4/Mn3O4/rGO is mainly attributed to the synergistic effects of Fe3O4, Mn3O4, rGO, and Oxone.

The planar structure of the designed α-MnO2–NH2–RGO hybrid was prepared and characterized and used to remove hexavalent chromium ions (Cr6+) from aqueous solutions. The characterization of the novel adsorbent was carried out using the Fourier transform infrared spectrum (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and the Brunauer–Emmett–Teller (BET) theory. Cr6+ adsorption efficiency was investigated as a function of the pH of Cr6+ solution, contact time and temperature. The results showed that the adsorption capacity strongly depended on the pH and that the adsorption equilibrium data were best described by the Freundlich isothermal model. The maximum sorption capacity towards Cr6+ was 371 mg g−1. The kinetic adsorption was fitted to the pseudo-second-order kinetics model and it indicated that the adsorption mechanism is physical or chemical sorption on heterogeneous materials. The thermodynamic parameters were obtained, and the results showed that the adsorption process is spontaneous and exothermic. The adsorption capacity of α-MnO2–NH2–RGO can remain as high as 81% after five usage cycles. As a result, these findings propose that α-MnO2–NH2–RGO could be used as an outstanding adsorbent for Cr6+.

Poly(m-phenylenediamine)-coated Fe3O4/o-MWCNTs nanoparticles (PmPD/Fe3O4/o-MWCNTs) were synthesized by one-step chemical oxidation polymerization. The materials were characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometry, and Brunauer–Emmett–Teller surface area measurement. Hexavalent chromium adsorption by PmPD/Fe3O4/o-MWCNTs was found to be strongly dependent on the solution pH. The adsorption isotherm of Cr(VI) onto PmPD/Fe3O4/o-MWCNTs fitted the Langmuir isotherm model at different temperatures and the maximum adsorption capacity was determined to be 346 mg g−1. The adsorption process of different initial concentrations can be described by the pseudo-second-order kinetic model. The intraparticle diffusion study revealed that external mass transfer and intraparticle diffusion apparently influenced the overall rate process. The values of thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic. In addition, the adsorption mechanism included both the physical and the chemical adsorption mechanisms. After adsorption, PmPD/Fe3O4/o-MWCNTs could be conveniently separated from the media by an external magnetic, and the adsorption capacity can remain at up to 52% after five times of usage. These results demonstrate that PmPD/Fe3O4/o-MWCNTs are a good candidate for the removal of Cr(VI) from aqueous solution.