MIL-101(Fe)-based catalysts have been widely used for degradation of organic pollutants based on peroxymonosulfate (PMS) activation. Hence, a facile calcination and hydrothermal method was used in this study to prepare a MIL-101(Fe)/g-C₃N₄ composite catalyst with high activity and high stability for PMS activation to degrade tetracycline hydrochloride (TC) under visible-light irradiation. We clearly elucidated the mechanism involved in the MIL-101(Fe)/g-C₃N₄ photo Fenton-catalyzed PMS activation process by separating the PMS activation and pollutant oxidation processes. The synergetic effects of MIL-101(Fe) and g-C₃N₄ involved MIL-101(Fe) acting as an electron shuttle mediating electron transfer from the organic substrate to PMS, accompanied by redox cycling of the surface Fe(II)/Fe(III). Multiple experimental results indicated that PMS was bound to the surface of MIL-101(Fe)/g-C₃N₄ during visible irradiation and generation of sulfate radicals (SO₄^•-), hydroxyl radicals (•OH) and superoxide anion free radicals (•O₂^-) for the radical pathway and singlet oxygen (¹O₂) and holes (h⁺) for the nonradical pathway. The major degradation pathways for TC can be described as demethylation, deamination, deamidation and carbonylation. This work provides valuable information and advances the fundamental understanding needed for design and syntheses of metal-free conjugated polymers modified by metal-organic frameworks for heterogeneous photo-Fenton reactions.

The photocatalytic degradation of organic pollutants is an effective method of controlling environmental pollution. ZnO nanoparticles (ZnO NPs) were prepared by the solvothermal method and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–visible diffuse reflectance spectroscopy (UV–Vis DRS). The results showed that the ZnO NPs had a uniform size of 25–40 nm, hexagonal wurtzite structure, and a band gap of 2.99 eV. The photocatalytic degradation of methyl orange (MO) and p-nitrophenol (PNP) was used as a model reaction to evaluate the photocatalytic activity of ZnO NPs. The photocatalytic degradation rates (pseudo-first-order kinetics) of MO and PNP were 92% (0.0128 min⁻¹) and 56.2% (0.0042 min⁻¹), respectively, with a 25 W ultraviolet lamp, MO/PNP concentration = 20 mg/L, ZnO NPs dose = 1.5 g/L, and time = 180 min. The photocatalytic mechanism of ZnO NPs and degradation pathways of MO and PNP were also proposed. The results provide valuable information and guidance for the treatment of wastewater via photocatalytic methods.

In order to improve the catalytic activity of cobalt molybdate (CoMoO₄), a PDS-activated and UV-vis assisted system was constructed. CoMoO₄ was prepared by coprecipitation and calcination, and characterized by XRD, FTIR, Raman, SEM, TEM, XPS, TGA Zeta potential, BET, and UV-Vis DRS. The results showed that the morphology of the CoMoO₄ nanolumps consisted of stacked nanosheets. XRD indicated the monoclinic structures with C2/m (C³_2h, #12) space group, which belong to α-CoMoO₄, and both Co²⁺ and Mo⁶⁺ ions occupy distorted octahedral sites. The pH of the isoelectric point (pHIEP) of CMO-8 at pH = 4.88 and the band gap of CoMoO₄ was 1.92 eV. The catalytic activity of CoMoO₄ was evaluated by photo-Fenton degradation of Congo red (CR). The catalytic performance was affected by calcination temperature, catalyst dosage, PDS dosage, and pH. Under the best conditions (0.8 g/L CMO-8, PDS 1 mL), the degradation efficiency of CR was 96.972%. The excellent catalytic activity of CoMoO₄ was attributed to the synergistic effect of photo catalysis and CoMoO₄-activated PDS degradation. The capture experiments and the ESR showed that superoxide radical (·O₂⁻), singlet oxygen (¹O₂), hole (h⁺), sulfate (SO₄⁻·), and hydroxyl (·OH⁻) were the main free radicals leading to the degradation of CR. The results can provide valuable information and support for the design and application of high-efficiency transition metal oxide catalysts.

The environmentally benign Fe₂(MoO₄)₃ plays a crucial role in the transformation of organic contaminants, either through catalytically decomposing oxidants or through directly oxidizing the target pollutants. Because of their dual roles and the complex surface chemical reactions, the mechanism involved in Fe₂(MoO₄)₃-catalyzed PDS activation processes remains obscure. In this study, Fe₂(MoO₄)₃ was prepared via the hydrothermal and calcine method, and photoFenton degradation of methyl orange (MO) was used to evaluate the catalytic performance of Fe₂(MoO₄)₃. Fe₂(MoO₄)₃ catalysts with abundant surface oxygen vacancies were used to construct a synergistic system involving a photocatalyst and PDS activation. The oxygen vacancies and Fe²⁺/Fe³⁺ shuttle played key roles in the novel pathways for generation of •O₂⁻, h⁺, and ¹O₂ in the UV–Vis + PDS + FMO-6 photoFenton system. This study advances the fundamental understanding of the underlying mechanism involved in the transition metal oxide-catalyzed PDS activation processes.

Ambient air quality is a complex dynamical system that is affected by a number of subsystems, such as particulate matter emission, meteorological factors, and socioeconomic factors. However, the mechanism of action of meteorological factors and socioeconomic factors and correlation between cities are unclear. The spearman correlation coefficient was used to analyze and explore the spatial and temporal differentiation of air quality (SO₂, NO₂, PM10, PM2.5, CO-95per and O_3–8h), the proportion of days with heavy pollution (PDHP), the proportion of days without pollution (PDWP) and influencing factors (particulate matter emission, meteorological factors and socioeconomic factors) in 16 cities in Shandong Province in the period 2019–2020. The results indicated that the concentrations of SO₂, NO₂, PM10, PM2.5 and CO-95per showed a trend of “winter high, summer low”, while the concentration of O_3–8h showed a trend of “winter low, summer high”. The air quality of coastal cities was better than that of inland cities. Temperature and precipitation were negatively correlated with the concentrations of SO₂, NO₂, PM10, PM2.5, CO-95per (P < 0.05). There were significant positive correlations between SO₂ and the proportion of secondary industry, between PM10 and population density, and between PM2.5 and population density. The annual mean concentrations of PM2.5, PM10, NO₂ and SO₂ were positively correlated, and the positive correlation between PM10 and PM2.5 was extremely significant. The correlation between pollutants in the heating period and nonheating period was significantly different. PM2.5/O_3–8h and PM2.5/NO₂ had no correlation in the heating period but had a significant positive correlation in the nonheating period. The PDHP in the heating period was positively correlated with SO₂, NO_2, PM10, PM2.5 and CO-95per, indicating that the air pollutants released during the heating process in winter are important factors that affect the air quality. There was a significant positive correlation between different cities in Shandong Province, and the correlation was affected by the distance between cities and the type of pollutants.