Haibo Liu

Fe(III) is widely present in non-stoichiometric pyrrhotite (Fe_1−xS, 0 < x < 0.125); however, its role in determining the catalytic activity of Fe_1−xS remains unclear. Herein, Fe_1−xS with varying Fe(III) were prepared via the sulfidation of goethite and the pyrolysis of pyrite at different temperatures, respectively, and subsequently employed to activate peroxydisulfate (PDS) for tetracycline (TC) removal. A combination of characterization and chemical analysis revealed that the Fe(III) content in sulfurized Fe_1−xS and pyrolytic Fe_1−xS decreased linearly with increasing sulfidation temperature (R² = 0.9875) and calcination temperature (R² = 0.9665), respectively. Electrochemical and zeta potential measurements showed that both the electron-transfer capability and surface electronegativity of Fe_1−xS increased with increasing Fe(III) content. Importantly, in the Fe_1−xS/PDS system, the Fe(III) content in sulfurized Fe_1−xS and pyrolytic Fe_1−xS was positively correlated with the second-order reaction rate constant (k₂, R² = 0.9276) and first-order reaction rate constant (k₁, R² = 0.9741) for TC removal, respectively, suggesting a clear structure-activity relationship between Fe(III) content and catalytic activity. Meanwhile, Fe release, reactive oxygen species (ROSs) generation, and PDS consumption increased with increasing Fe(III) content in Fe_1−xS. In addition, sulfurized Fe_1−xS and pyrolytic Fe_1−xS show a wide pH operating range and moderate reusability. These findings provide valuable insights into the rational design of Fe_1−xS-based catalysts for advanced oxidation processes to remove refractory organic pollutants.

Herein, we tailored a series of ultrathin MnO₂ nanolayers coated on the surface of commercial goethite (α-FeOOH) by a facile in situ chemical precipitation method. α-FeOOH inhibited the MnO₂ crystal growth via the incorporation of K⁺ ions between MnO₂ and α-FeOOH interfaces during the synthesis process. The hybrid design of MnO₂ with an ultrathin nanolayer structure could reduce the electron transfer resistance and bring abundant oxygen vacancies, accelerating the activation of molecular O₂ to generate more oxygen-free radical species and favoring the thermodynamic HCHO oxidation. The ROS quenching in gas/aqueous systems and DRIFTS results demonstrated that ^•O₂^- was responsible for HCHO oxidization, which assisted the preliminary intermediate dioxymethylene dehydrogenation into formate species. The 25%MnO₂@FeOOH(25wt% of MnO₂) catalyst was subsequently loaded into the filter substrates of a commercial air cleaner and tested in an indoor room with actual application conditions. As a result, the composite filter could eliminate different initial concentrations of HCHO (150-450 ppb) to the WHO guideline value (≈81 ppb) within 60 min. Furthermore, the 25%MnO₂@FeOOH sample was also effective against the representative bacteria and mold in indoor air. This study provides new insight into the role of the chemisorbed ROS for HCHO oxidation at ambient temperature.

Despite the biogeochemical importance of phosphate fate and transport in aquatic environments, little is known about how competition with other common aqueous oxyanions affects its retention by mineral surfaces. Here, we examined the competitive uptake of phosphate and silicate on goethite over a wide pH range, using batch measurements supported by DFT calculations. The results show selective adsorption of phosphate at pH < 4 and silicate at pH > 10 with little to no competitive effect. However, between 4 < pH < 10, the total phosphate and silicate loading was found to be almost equal to that of silicate loading from single-component solution, revealing a proportionate competition for surface site types and a competitive effect controlling their mutual retention. DFT-calculated adsorption energies and charge density redistributions for various surface complexes on different charged (101) and (210) facets are consistent with the trends observed in batch measurements, suggesting that the observed behavior reflects the primary controlling influence of goethite surface chemistry at the molecular scale. An important implication is that at the circumneutral pH in most environmental systems, where iron oxyhydroxides comprise much of the reactive interfacial area, unbound phosphate concentrations may be strongly controlled by dissolved silicate concentration, and vice versa.