Effective peroxymonosulfate activation by natural molybdenite for enhanced atrazine degradation: Role of sulfur vacancy, degradation pathways and mechanism.

In this study, natural molybdenite (MoS2) was applied to activate peroxymonosulfate (PMS) for the removal of atrazine (ATZ) and its degradation mechanism was investigated. Molybdenite exhibits superior catalytic performance. The best condition for atrazine degradation efficiency (>99%) was obtained with molybdenite concentration of 0.4 g/L, PMS concentration of 0.1 mM, and ATZ concentration of 12 µM within 10 min under experimental conditions. Electron paramagnetic resonance (EPR) test and chemical probe test further proved that HO• and SO4•- played important roles in the molybdenite/PMS system, and SO4•- was dominant. Meanwhile, Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) tests showed that sulfur vacancies and edge sulfur played important roles in the system. Edge sulfur was conducive to Mo4+ exposure, while sulfur vacancy facilitated electron transfer and reduced Mo6+ back to Mo4+. Combined with DFT calculation, the role of sulfur in the degradation process was verified. Besides, five ATZ degradation pathways were proposed. Finally, the degradation ability of the molybdenite/PMS system for different pollutants and in actual water bodies was also explored. This work provided ideas for exploring the degradation of organic contaminants by natural minerals.

Insights into the role of in-situ and ex-situ hydrogen peroxide for enhanced ferrate(VI) towards oxidation of organic contaminants.

Recently, several studies have been conscious of the promotion effect of hydrogen peroxide (H2O2), a self-decay product of ferrate (Fe(VI)), on Fe(VI) to oxidize contaminations, but the pivotal activation mechanism has not been thoroughly evaluated. This work aims to compare and reveal the promoting mechanism of H2O2 in Fe(VI) and Fe(VI)-H2O2 processes, and to illustrate the practical use potential of Fe(VI)-H2O2 system. Many lines of evidence verified the involvement of •OH and O2•- in pollutant degradation were excluded in Fe(VI) and Fe(VI)-H2O2 systems, meaning that high dosage of H2O2 cannot trigger an activation pathway different from in-situ H2O2. The better oxidation performance of the Fe(VI)-H2O2 system than Fe(VI) alone was ascribed to the catalytic role of in-situ and ex-situ H2O2, which can directly and/ or indirectly facilitate the formation of Fe(IV) and Fe(V). Considering the structural similarity of peroxymonosulfate (PMS) and peroxydisulfate (PDS) with H2O2 as well as their universality in water pollutant remediation, the oxidation properties and reactive oxidants of Fe(VI)-PMS and Fe(VI)-PDS processes were also examined. Besides, the Fe(VI)-H2O2 system suffered from less restriction by inorganic ions and natural organic matter, and exhibited satisfactory pollutant removal effects in real water. Overall, this work provides a further and comprehensive cognition about the role of H2O2 in Fe(VI) and Fe(VI)-H2O2 systems.

Insights into the Electron-Transfer Mechanism of Permanganate Activation by Graphite for Enhanced Oxidation of Sulfamethoxazole.

Many reagents as electron sacrificers have been recently investigated to induce decomposition of permanganate (KMnO4) to produce highly reactive intermediate Mn species toward oxidation of organic contaminants; however, this strategy meanwhile causes low KMnO4 utilization efficiency. This study surprisingly found that graphite can mediate direct electron transfer from organics (e.g., sulfamethoxazole (SMX)) to KMnO4, resulting in high KMnO4 utilization efficiency, rather than reductive sites of graphite-induced conversion of KMnO4 to highly reactive intermediate Mn species. The galvanic oxidation process (GOP) and comparative experiments of different organic contaminants prove that the KMnO4/graphite system mainly extracts electrons from organic contaminants via a one-electron pathway instead of a two-electron pathway. More importantly, the KMnO4/graphite system has superior reusability, graphite can keep a long-lasting reactivity, and the KMnO4 utilization efficiency elevates significantly after each cycle of graphite. The transformation of SMX in the KMnO4/graphite system mainly includes self-coupling, hydroxylation, oxidation, and hydrolytic reaction. The work will improve insights into the electron-transfer mechanism and unveil the advantages of efficient KMnO4 utilization in the KMnO4-based technologies in environmental remediation.

Critical review of natural iron-based minerals used as heterogeneous catalysts in peroxide activation processes: Characteristics, applications and mechanisms.

Recently, an increasing number of works have been reported about iron-based materials applied as catalysts in peroxide activation processes to degrade pollutants in water. Iron-based catalysts include synthetic and natural iron-based materials. However, some synthetic iron-based materials are difficult to scale up in the practical applications due to high cost and serious secondary environmental pollution. In contrast, natural iron-based minerals are more available and cheaper, and also hold a great promise in peroxide activation processes for pollutant degradation. In this review, we classify different natural iron-based materials into two categories: iron oxide minerals (e.g., magnetite, hematite, and goethite,), and iron sulfide minerals (e.g., pyrite and pyrrhotite,). Their overview applications in peroxide activation processes for pollutant degradation in wastewaters are systematically summarized for the first time. Moreover, the peroxide activation mechanisms induced by natural minerals, and the influences of reaction conditions in different systems are discussed. Finally, the application prospects and existing drawbacks of natural iron-based minerals in the peroxide activation processes for wastewater treatment are proposed. We believe this review can shed light on the application of natural iron-based minerals in peroxide activation processes and present better perspectives for future researches.

Metal-free black-red phosphorus as an efficient heterogeneous reductant to boost Fe3+/Fe2+ cycle for peroxymonosulfate activation.

In this work, a novel metal-free black-red phosphorus (BRP) was prepared from red phosphorus (RP) and applied in Fe2+/peroxymonosulfate (PMS) process. Compared with that of RP, the contaminant degradation performance of BRP was significantly elevated due to the enhanced electron transfer from BRP to Fe3+. This enhancement was mainly induced by size decrease effect, the removal of oxidation layer and the partial phase conversion. Moreoevr, BRP avoided the radical quenching reaction caused by reductant itself, whereas it was inevitable using homogeneous reductant like hydroxylamine. More importantly, the system had a superior recyclability and strong resistance to natural water. Though concurrent side-reaction between PMS and BRP occured, multiple PMS dosage could remarkedly alleviated the side-reaction, thus elevating PMS utilization efficiency. The dominant BRP oxidation products included phosphite and phosphate. Interestingly, moderate increase of Fe3+ concentration could efficiently reduce the by-product formation via the prompt PMS activation by regenerated Fe2+. Our work clarified the acceleration mechanism of Fe3+/Fe2+ cycle by BRP and proposed the control strategy of by-prodoct formation.

Activation CuFe2O4 by Hydroxylamine for Oxidation of Antibiotic Sulfamethoxazole.

A new potential oxidation process is provided by CuFe2O4/hydroxylamine (HA) system for degradation of antibiotics in water. The CuFe2O4/HA system can generate reactive oxygen species (ROS) for the degradation of sulfamethoxazole (SMX). The addition of radical scavengers, including benzoquinone (BQ) and catalase (CAT), inhibited the oxidation of SMX in CuFe2O4/HA system. Electron transfer in the CuFe2O4/HA system played a key function for the generation of ROS and the degradation of SMX. The main ROS, was the superoxide radical (O2•-) mainly generated from adsorbed oxygen (O2(A)), which came from the oxidation of the lattice oxygen (O2-(L)) in CuFe2O4. The CuFe2O4/HA system was effectively applicable for a broad pH range (approximately 5-10). In addition, the activation mechanism for CuFe2O4/HA system was studied with the target contaminant SMX. Finally, the degradation pathways of SMX were proposed under the optimal conditions in CuFe2O4/HA system.

Premagnetization enhancing the reactivity of Fe0/(passivated Fe0) system for high concentration p-nitrophenol removal in aqueous solution.

In order to strengthen the treatment efficiency of Fe0 based system for high concentration wastewater treatment, Fe0 particles were passivated by concentrated nitric acid, and a premagnetization Fe0/(passivated Fe0) system was setup for high concentration p-nitrophenol (PNP) removal in this study. The significant parameters of this system were optimized. Under the optimal conditions, the premagnetization Fe0/(passivated Fe0) system could obtain high kobs value for PNP removal (0.100 min-1) and COD removal (15.0% after 60 min) for high concentration PNP (500 mg/L) treatment. In addition, five control experiments were set up to confirm the advantage of the premagnetization Fe0/(passivated Fe0) system. The results suggest that passivated Fe0 particles could be stimulated better than Fe0 particles by premagnetization process, and the premagnetization Fe0/(passivated Fe0) systems is much superior to the other five control systems. Furthermore, the pathway for PNP destruction treated by 6 different systems was also proposed according to intermediates determination by High Performance Liquid Chromatography (HPLC) and UV-vis spectrum, and the carbon mass balance was demonstrated according to the COD and HPLC analyses. Finally, the characteristics of (premagnetization) Fe0 and passivated Fe0 was detected by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and vibrating sample magnetometer (VSM), and the mechanism of premagnetization effectively enhancing the reactivity of Fe0/(passivated Fe0) system (better than that of Fe0 system) was proposed. Consequently, the premagnetization for reactivity improvement of Fe0/(passivated Fe0) system is a promising technology to enhance the efficiency of this system for high concentration wastewater treatment.

Electrolysis assisted persulfate with annular iron sheet as anode for the enhanced degradation of 2, 4-dinitrophenol in aqueous solution.

Annular iron sheet (AIS), playing a dual role of anode electrode and source of activator, was combined with electrolysis technology for the activation of persulfate to improve the degradation of 2, 4-dinitrophenol (DNP) in aqueous solution. In this study, effects of current density (0-10.0mA/cm2), persulfate (PS) dosage (0-8.0mM), initial pH (3.0-11.0), reaction temperature (25-60°C) and reaction time (0-30min) on COD removal of DNP in aqueous solution were investigated, respectively. COD removal reached its maximal value (63.4%) after 15min treatment due to the synergistic effect in electro/AIS/PS system under the optimal conditions. Furthermore, comparative studies of 7 different experimental processes were setup. In addition, the reasonable DNP degradation pathway was proposed based on intermediates detected by HPLC. According to characterization analysis of SEM-EDS, XRD and XPS of the generated flocculation in electro/AIS/PS system, the possible reaction mechanism was proposed in detail. In a word, the electrolysis process coupled with annular iron sheet as anode activating persulfate technology shows a significant synergetic effect in enhancing degradation of DNP in aqueous solution.