Mn2O3/Mn3O4-Cu1.5Mn1.5O4 spinel as an efficient Fenton-like catalyst activating persulfate for the degradation of bisphenol A: Superoxide radicals dominate the reaction

Electron transfer mediated photo-Fenton-like synergistic catalysis of Fe,Cu-doped MIL-101 coupled with Ag3PO4: Quantitative evaluation and DFT calculations

Widespread use of tetracycline (TC) results in its persistent residue and bioaccumulation in aquatic environments, posing a high toxicity to non-target organisms. In this study, a bimetal-doped composite material Ag3PO4/MIL-101(Fe,Cu) has been designed for the treatment of TC in aqueous solutions. As the molar ratio of Fe/Cu in composite is 1:1, the obtained material AP/MFe1Cu1 is placed in an aqueous environment under visible light irradiation in the presence of 3 mM peroxydisulfate (PDS), which forms a photo-Fenton-like catalytic system that can completely degrade TC (10 mg/L) within 60 min. Further, the degradation rate constant (0.0668 min-1) is 5.66 and 7.34 times higher than that of AP/MFe and AP/MCu, respectively, demonstrating a significant advantage over single metal-doped catalysts. DFT calculations confirm the strong adsorption capacity and activation advantage of PDS on the composite surface. Therefore, the continuous photogenerated electrons (e-) accelerate the activation of PDS and the production of SO4•-, resulting in the stripping of abundant photogenerated h + for TC oxidation. Meanwhile, the internal circulation of FeⅢ/FeⅡ and CuⅡ/CuⅢ in composite also greatly enhances the photo-Fenton-like catalytic stability. According to the competitive dynamic experiments, SO4•- have the greatest contribution to TC degradation (58.93%), followed by 1O2 (23.80%). The degradation intermediates (products) identified by high-performance liquid chromatography-mass spectrometry (HPLC/MS) technique indicate the involvement of various processes in TC degradation, such as dehydroxylation, deamination, N-demethylation, and ring opening. Furthermore, as the reaction proceeds, the toxicity of the intermediates produced during TC degradation gradually decreases, which can ensure the safety of the aquatic ecosystem. Overall, this work reveals the synergy mechanism of PDS catalysis and photocatalysis, as well as provides technical support for removal of TC-contaminated wastewater.

Surfactant enhanced persulfate system for the synergistic oxidation and reduction of mixed chlorinated hydrocarbons

Surfactant-enhanced in-situ chemical oxidation (S-ISCO) is widely applied in soil and groundwater remediation. However, the role of surfactants in the reactive species (RSs) transformation remains inadequately explored. This work introduced nonionic surfactant Tween-80 (TW-80) into a nano zero-valent iron (nZVI) activated persulfate (PS) system. The findings indicate that PS/nZVI/TW-80 system can realize the concurrent removal of trichloroethylene (TCE), tetrachloroethene (PCE), and carbon tetrachloride (CT), whereas CT cannot be eliminated without TW-80 presence. Further analysis unveiled that hydroxyl (HO•) and sulfate radicals (SO4－•) were the primary species for TCE and PCE degradation, while CT was reductively eliminated by surfactant radicals generated from TW-80. Moreover, the surfactant radicals were found to accelerate Fe(III)/Fe(II) cycle, reduce the production of iron sludge, and increase PS decomposition. The possible degradation routes of mixed chlorinated hydrocarbons (CHCs) and the decomposition pathways of TW-80 were proposed through the density function theory (DFT) calculation and intermediates analysis. Additionally, the effects of other nonionic surfactants on the simultaneous removal of TCE, PCE, and CT, and the practical applications using the actual contaminated groundwater were also evaluated. This study provides theoretical support for the simultaneous removal of CHCs, particularly those containing perchlorinated contaminants, using the S-ISCO techniques.

Sulfite activation by Jahn-Teller-driven oxygen vacancies Cu-Mn composite oxide for chlortetracycline degradation

Copper-manganese composite metal oxides (CuMnOy) were prepared by hydrolysis-driven oxidation-reduction method and used to activate sulfite to degrade chlortetracycline hydrochloride (CTC) for the first time. The Jahn-Teller ions Mn3+ and Cu2+ exist in CuMnOy, which form a solid electric charge transport redox system and ensure the continuous generation of reactive oxygen species (ROS). Through the systematic study of the experimental parameters such as sulfite concentration, catalyst metal molar ratio, catalyst amounts and initial pH, the optimal degradation rate of CTC could reach 91.74% within 10 min and 94.46% after 30 min. The major reactive radicals were determined by radical quenching experiments and electron paramagnetic resonance (EPR) trapping techniques, and it was confirmed that SO4•- and •O2- played a nonnegligible role in the process of degrading CTC. Density functional theory (DFT) calculations show that higher Fukui indices (f- and f0) of CTC sites are more vulnerable to free radical attack. CuMnOy has low CTC degradation intermediate toxicity, high catalytic performance, good anti-interference ability, reusability and stability, and possesses decent application potential in the actual water treatment field.

S modified manganese oxide for high efficiency of peroxydisulfate activation: Critical role of S and mechanism

Mn₂O₃ as a typical Mn based semiconductor has attracted growing attention due to its peculiar 3d electron structure and stability, and the multi-valence Mn on the surface is the key to peroxydisulfate activation. Herein, an octahedral structure of Mn₂O₃ with (111) exposed facet was synthesized by a hydrothermal method, which was further sulfureted to obtained a variable-valent Mn oxide for the high activation efficiency of peroxydisulfate under the light emitting diode irradiation. The degradation experiments showed that under the irradiation of 420 nm light, S modified manganese oxide showed an excellent removal for tetracycline within 90 min, which is about 40.4% higher than that of pure Mn₂O₃. In addition, the degradation rate constant k of S modified sample increased 2.17 times. Surface sulfidation not only increased the active sites and oxygen vacancies on the pristine Mn₂O₃ surface, but also changed the electronic structure of Mn due to the introduce of surface S^2-. This modification accelerated the electronic transmission during the degradation process. Meanwhile, the utilization efficiency of photogenerated electrons was greatly improved under light. Besides, the S modified manganese oxide had an excellent reuse performance after four cycles. The scavenging experiments and EPR analyses showed that •OH and ¹O₂ were the main reactive oxygen species. This study therefore provides a new avenue for further developing manganese-based catalysts towards high activation efficiency for peroxydisulfate.

The debatable role of singlet oxygen in persulfate-based advanced oxidation processes

Singlet oxygen (¹O₂) attracts much attention in persulfate-based advanced oxidation processes (PS-AOPs), because of its wide pH tolerance and high selectivity toward electron-rich organics. However, there are conflicts about the ¹O₂ role in PS-AOPs on several aspects, including the formation of different key reactive oxygen species (ROS) at similar active sites, pH dependence, broad-spectrum activity, and selectivity in the elimination of organic pollutants. To a large degree, these conflicts root in the drawbacks of the methods to identify and evaluate the role of ¹O₂. For example, the quenchers of ¹O₂ have high reactivity to other ROS and persulfate as well. In addition, electron transfer process (ETP) also selectively oxidizes organics, having a misleading effect on the identification of ¹O₂. Therefore, in this review, we summarized and discussed some basic properties of ¹O₂, the debatable role of ¹O₂ in PS-AOPs on multiple aspects, and the methods and their drawbacks to identify and evaluate the role of ¹O₂. On the whole, this review aims to better understand the role of ¹O₂ in PS-AOPs and further help with its reasonable utilization.

Insight into mechanism of peroxydisulfate activation by natural pyrite: Participation of Fe(IV) and regulation of Fe(III)/Fe(II) cycle by sulfur species

In this study, natural pyrite (NP) was used to activate peroxydisulfate (PDS) for imidacloprid (IMD) degradation. NP was characterized by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Effects of key reaction parameters (NP dosage, PDS concentration and initial pH) and co-existing ions on IMD degradation in the NP/PDS system were investigated. Quenching experiments and electron spin resonance (ESR) tests identified the existence of sulfate radical (SO₄^•-), hydroxyl radical (^•OH), singlet oxygen (¹O₂) and superoxide radical (O₂^•-). The cumulative concentration of SO₄^•- and ^•OH were quantified by the formation of benzoquinone (BQ) and p-hydroxybenzoic acid (HBA), respectively. Meanwhile, more than 60% of methylphenyl sulfoxide (PMSO) was selectively converted to methylphenyl sulfone (PMSO₂), revealing that Fe(IV) was dominant in the NP/PDS system. The order of contribution of the three reactive species in the NP/PDS system was Fe(IV) > ^•OH > SO₄^•- (contributions of ¹O₂ and O₂^•- were negligible). Fe(II) released from NP played a crucial role in PDS activation, and sulfur species in NP could also boost Fe(III)/Fe(II) cycle and contribute to the generation of reactive species. Further, the possible degradation pathways of IMD have been proposed based on the detected intermediates using high-performance liquid chromatography-mass spectrometry (HPLC-MS), and the toxicity (including acute toxicity, developmental toxicity and mutagenicity) of these intermediates have been predicted using Toxicity Estimation Software Tool (T.E.S.T). Moreover, NP/PDS system was applied in four natural water bodies and IMD degradation efficiency reached more than 97% after adjusting the pH to 3. The fluorescence excitation-emission matrix (EEM) spectra showed that in addition to IMD, NP/PDS system could also remove other impurities.