In this study, different crystalline structures of FeOOH have been prepared. α-FeOOH was synthesized through a hydrothermal method, whereas β-FeOOH was synthesized via a direct hydrolysis method. Moreover, γ- and δ-FeOOH were prepared by precipitation methods through slow and quick oxidation, respectively. On this basis, their crystal structure, morphology, and surface area were measured. Then, all the synthesized materials were applied to activate peroxymonosulfate (PMS) to generate sulfate radicals (SO₄ ⁻˙) for acid orange 7(AO7) degradation. Compared with α-FeOOH, β-FeOOH, and γ-FeOOH, δ-FeOOH showed more efficient decolorization of AO7 in the catalytic system because of its abundant surface area and crystalline structure. The effects of several parameters in the δ-FeOOH/PMS/AO7 system were investigated. The results show that the initial pH, which is related to the features of surface hydroxyl groups, is the decisive factor, and excellent catalytic activity is maintained in the pH range 5–8. The increase of catalyst dosage and appropriate increase of PMS concentration contributed to promote the degradation effect. However, self-quenching was observed in a high PMS concentration system. Moreover, δ-FeOOH was stable after six consecutive cycles, and the leaching of iron ions was negligible. According to the quenching test and electron spin resonance analysis, both SO₄ ⁻˙ and ˙OH were the dominant radicals for AO7 degradation.

In this study, we proposed a one-pot, low-cost and environmentally-friendly approach for fabrication of tunable Fe₃O₄–MnO₂ core–shell nanoplates at ambient temperature and pressure. The results of high resolution transmission electron microscope (HRTEM) and energy dispersive X-ray (EDX) spectra showed that Fe₃O₄ nanoplate cores were coated by amorphous MnO₂ shells. In addition, the morphology of the as-synthesized nanoparticles could be controlled through manipulating the molar ratio of FeSO₄ and KMnO₄. The increased KMnO₄ dosage led to the decrease in thickness of the nanoplate cores from 12 nm to <5 nm, the increase in edge length from 40 nm to 70 nm, and the increase in thickness of the shell from <1 nm to 5 nm. In addition, the formation mechanism of Fe₃O₄–MnO₂ core–shell nanoplates was proposed based on all the observations at arbitrary molar ratio of Fe and Mn precursors. Last, the performance of sample 2 was examined as an absorbent for removal of arsenic anions from aqueous solution, and the results showed high removal capacity for both arsenite and arsenate. Moreover, the synthesized NPs have been demonstrated to be capable of enhancing the adsorption of arsenite through simultaneous oxidation of arsenite to arsenate by MnO₂.

An energy-effective one-pot method was proposed to fabricate Ag–Fe₃O₄ nanocomposites through the redox reaction between Ag₂O and Fe(OH)₂ in the absence of additional reductant. This new nanomaterial exhibits potent antibacterial properties and can be separated easily by applied magnetic field.

The proposed mechanism for the generation of ˙OH and degradation of 4-CP in the Au–Fe₃O₄/H₂O₂system.