Effective activation of peroxymonosulfate with natural manganese-containing minerals through a nonradical pathway and the application for the removal of bisphenols

Effect of interaction between dissolved organic matter and iron/manganese (hydrogen) oxides on the degradation of organic pollutants by in-situ advanced oxidation techniques

Iron and manganese (hydrogen) oxides (IMHOs) exhibit excellent redox capabilities for environmental pollutants and are commonly used in situ chemical oxidation (ISCO) technologies for the degradation of organic pollutants. However, the coexisting dissolved organic matter (DOMs) in surface environments would influence the degradation behavior and fate of organic pollutants in IMHOs-based ISCO. This review has summarized the interactions and mechanisms between DOMs and IMHOs, as well as the properties of DOM-IMHOs complexes. Importantly, the promotion or inhibition impact of DOM was discussed from three perspectives. First, the presence of DOMs may hinder the accessibility of active sites on IMHOs, thus reducing their efficiency in degrading organic pollutants. The formation of compounds between DOMs and IMHOs alters their stability and activity in the degradation process. Second, the presence of DOMs may also affect the generation and transport of active species, thereby influencing the oxidative degradation process of organic pollutants. Third, specific components within DOMs also participate and affect the degradation pathways and rates. A comprehensive understanding of the interaction between DOMs and IMHOs helps to better understand and predict the degradation process of organic pollutants mediated by IMHOs in real environmental conditions and contributes to the further development and application of IMHO-mediated ISCO technology.

Treatment of bisphenol pollutant in water by N,P-co-doped carbon nanosheet: Fast degradation, toxicity elimination and reaction mechanism investigation

Metal-free carbon catalysts perform well in peroxymonosulfate-based advanced oxidation process for the treatment of organic pollutant-containing wastewater. Herein, a natural biomolecule of adenosine triphosphate (ATP), containing abundant N and P elements, served as sole precursor to prepare N,P-co-doped carbon through one-step anoxic pyrolysis, which was applied as peroxymonosulfate activator to treat bisphenol-contaminated water. Owing to the endogenous N and P elements in ATP, in-situ doping was achieved for the prepared carbon material with excellent doping effect, such as high doping amount and numerous defects. During pyrolysis process, the generated gases facilitated the exfoliation of carbon structure, resulting in a nanosheet-like morphology with large specific surface area, e.g., 852.75 m² g^-1 for NPCN-900 sample obtained at 900 °C. Benefiting from the structural modulation brought by N,P co-doping, typical sample of NPCN-900 presented excellent catalytic performance towards bisphenol AF (BPAF) degradation through PMS activation. An apparent reaction rate constant of 0.4115 min^-1 was calculated under the investigated reaction conditions. Further studies indicated that ¹O₂, surface-bound •OH and SO₄^-• worked together in NPCN-900/PMS system for BPAF degradation. Graphitic N, pyrrolic N, CO groups, defect structure and the doped P atoms in NPCN-900 contributed to PMS activation. It was also important that the toxicity of BPAF solution could be preliminarily eliminated after treatment by NPCN-900/PMS system, which was verified by ecotoxicity assessments through ECOSAR program and green algae growth experiments.

Hydrothermally improved natural manganese-containing catalytic materials to degrade 4-chlorophenol

Natural manganese-containing mineral (NMM) was used as a catalyst in heterogeneous catalytic ozonation for 4-chlorophenol (4-CP) degradation. The surface and structural properties of NMM were modified by the hydrothermal aging process and called H-NMM. The catalytic activity of NMM and H-NMM were evaluated for the catalytic ozonation process (COP). The synergistic effect of NMM and H-NMM in ozonation processes for 4-CP degradation under optimal conditions (pH of 7, 1 g/L of NMM and H-NMM, 0.85 mg/min of O₃, and 15 min of reaction time) was measured by 3.04 and 4.34, respectively. During the hydrothermal process, Mn⁴⁺ and Fe²⁺ were converted to Mn²⁺ and Fe³⁺, which caused better performance of the H-NMM than the NMM. During the catalytic ozonation process, Mn²⁺ is completely oxidized, which increases the production of Hydroxyl radical (•OH). The reactive oxygen species (ROS) generated in the system were identified using radical scavenging experiments. •OH, superoxide radical (•O₂^-), and singlet oxygen (¹O₂) represented the dominant reactive species for 4-CP degradation. The O₃/H-NMM process indicated a powerful ability in the mineralization of 4-CP (66.31% of TOC degradation). H-NMM exhibited excellent stability and reusability in consecutive catalytic cycles, and the NMM exhibited desirable performance. This study offers NMM and H-NMM as effective, stable, and competitive catalysts for hastening and enhancing the ozonation process to mitigate environmentally related pollutants of high concern.

(NH4)2Mo3S13/MnFe2O4 hybrid with multiple active sites boosted activation of peroxymonosulfate for removal of tetracycline

Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) activation have attracted much attention in wastewater treatment. Here, a series of (NH₄)₂Mo₃S₁₃/MnFe₂O₄ (MSMF) composites were prepared and used as PMS activators to remove tetracycline (TC) for the first time. When the mass ratio of (NH₄)₂Mo₃S₁₃ to MnFe₂O₄ was 4.0 (MSMF4.0), the composite showed remarkable catalytic efficiency for activating PMS to remove TC. Over 93% of TC was removed in MSMF4.0/PMS system in 20 min. The aqueous ^•OH as well as the surface SO₄^•- and ^•OH were the primary reactive species for TC degradation in MSMF4.0/PMS system, and the comprehensive experimental results excluded the contributions of aqueous SO₄^•-, O₂^•-, and ¹O₂, high-valent metal-oxo species, and surface-bound PMS. The Mn(II)/Mn(III), Fe(II)/Fe(III), Mo(IV)/Mo(VI), and S^2-/SO_x^2- all contributed to the catalytic process. MSMF4.0 also showed excellent activity and stability after five cycles and significant degradation efficiency for a variety of pollutants. This work will provide theoretical basis for applying MnFe₂O₄-based composites in PMS-based AOPs.

Light and nitrogen vacancy-intensified nonradical oxidation of organic contaminant with Mn (III) doped carbon nitride in peroxymonosulfate activation

Recently, Mn-based materials have a great potential for selective removal of organic contaminants with the assistance of oxidants (PMS, H₂O₂) and the direct oxidation. However, the rapid oxidation of organic pollutants by Mn-based materials in PMS activation still presents a challenge due to the lower conversion of surface Mn (III)/Mn (IV) and higher reactive energy barrier for reactive intermediates. Here, we constructed Mn (III) and nitrogen vacancies (Nv) modified graphite carbon nitride (MNCN) to break these aforementioned limitations. Through analysis of in-situ spectra and various experiments, a novel mechanism of light-assistance non-radical reaction is clearly elucidated in MNCN/PMS-Light system. Adequate results indicate that Mn (III) only provide a few electrons to decompose Mn (III)-PMS* complex under light irradiation. Thus, the lacking electrons necessarily are supplied from BPA, resulting in its greater removal, then the decomposition of the Mn (III)-PMS* complex and light synergism form the surface Mn (IV) species. Above Mn-PMS complex and surface Mn (IV) species lead to the BPA oxidation in MNCN/PMS-Light system without the involvement of sulfate (SO₄^{• ̶}) and hydroxyl radicals (•OH). The study provides a new understanding for accelerating non-radical reaction in light/PMS system for the selective removal of contaminant.

Natural manganese sand activates sodium hypochlorite to enhance ionic organic contaminants removal: Optimization, modeling, and mechanism

Although sodium hypochlorite acting as an oxidant has been investigated for the role it plays in the degradation of organic contaminants, little attention has been paid to its activation and efficient utilization. In this study, natural manganese sand (NMS) was verified to be effective for activation of sodium hypochlorite (NaClO). Due to the generation of O₂^-, the removal efficiency of ionic organic contaminants in NMS/NaClO system was 1.9-4.1 times higher than that in NMS or NaClO alone. Hence, NMS activated NaClO system performed ~96.6 % contaminants removal efficiency at a wide pH range (pH 5-9). Kinetic modeling yielded that the NMS dosage was more important than NaClO dosage. Long-term stability was observed in the presence of various salts (bicarbonate, sulfate, phosphate, and chloride). Characterization results revealed that electron transfer among NMS, NaClO, and organic contaminants was responsible for NaClO activation. Then NaClO-based Fenton-like process was proposed by tracing the degradation intermediates of methyl orange (MO) and generations of reactive oxygen species in the MO/NMS/NaClO system. This study presents the potential of NMS to activate NaClO and enhance ionic organic contaminants removal from aquatic environments.

Fast peroxydisulfate oxidation of the antibiotic norfloxacin catalyzed by cyanobacterial biochar

Peroxydisulfate (PDS) is a common oxidant for organic contaminant remediation. PDS is typically activated by metal catalysts to generate reactive radicals. Unfortunately, as radicals are non-selective and metal catalysts may cause secondary contamination, alternative selective non-radical pathways and non-metal catalysts need attention. Here we investigated PDS oxidation of commonly detected antibiotic Norfloxacin (NOR) using cyanobacterial nitrogen rich biochars (CBs) as catalysts. NOR was fully degraded by CB pyrolysed at 950 °C (CB950) within 120 min. CB950 caused threefold faster degradation than low pyrolysis temperature (PT) CBs and achieved a maximum surface area normalized rate constant of 4.38 × 10^-2 min^-1 m^-2 L compared to widely used metal catalysts. CB950 maintained full reactivity after four repeated uses. High defluorination (82%) and mineralization (>82%) were observed for CB950/PDS. CBs were active over a broad pH range (3-10), but with twice as high rates under alkaline compared with neutral conditions. NOR is degraded by organic, ^•OH and SO₄^•- radicals in low PT CBs/PDS systems, where the presence of Mn^II promotes radical generation. Electron transfer reactions with radicals supplemented dominate high PT CBs/PDS systems. This study demonstrates high PT biochars from algal bloom biomass may find use as catalysts for organic contaminant oxidation.

Nonradical electron transfer-based peroxydisulfate activation by a Mn-Fe bimetallic oxide derived from spent alkaline battery for the oxidation of bisphenol A

Mn-Fe bimetallic oxide has been employed as an outstanding peroxydisulfate (PDS) activator, but the underlying mechanism is still controversial. In this work, Mn_0.27FeO_4.55 (MFBO) was synthesized using the recovered waste alkaline battery and its catalytic activity and mechanism for PDS activation were explored in detail. Results show that MFBO exhibited a higher catalytic activity than the individual single metal oxides (FeO_x and Mn₂O₃) due to the synergistic effect between Fe and Mn elements. The removal efficiency of bisphenol A (BPA) with an initial concentration of 10 mg/L reached 97.8% within 90 min in the presence of 0.5 g/L MFBO and 2.0 mM PDS. Moreover, the MFBO maintained high stability and reusability even after being recycled for five times. With the aid of a series of experiments and ex-situ/in-situ characterizations, a non-radical PDS activation mechanism was proposed, in which organic contaminants would be oxidized through a direct electron transfer pathway mediated by the metastable reactive complexes (MFBO-PDS*). Notably, the MFBO/PDS system revealed selective oxidation towards different organic pollutants and the reaction rates were closely related to their structures and properties. The research provided an effective alternation process for application of the waste battery, as well as developed a novel perspective for removal of recalcitrant aqueous contaminants through a nonradical mechanism.

Mn2O3 as an Electron Shuttle between Peroxymonosulfate and Organic Pollutants: The Dominant Role of Surface Reactive Mn(IV) Species

The environmentally benign Mn oxides play a crucial role in the transformation of organic contaminants, either through catalytically decomposing oxidants, e.g., peroxymonosulfate (PMS), or through directly oxidizing the target pollutants. Because of their dual roles and the complex surface chemical reactions, the mechanism involved in Mn oxide-catalyzed PMS activation processes remains obscure. Here, we clearly elucidate the mechanism involved in the Mn₂O₃ catalyzed PMS activation process by means of separating the PMS activation and the pollutant oxidation process. Mn₂O₃ acts as a shuttle that mediates the electron transfer from organic substrates to PMS, accompanied by the redox cycle of surface Mn(IV)/Mn(III). Multiple experimental results indicate that PMS is bound to the surface of Mn₂O₃ to form an inner-sphere complex, which then decomposes to form long-lived surface reactive Mn(IV) species, without the generation of sulfate radicals (SO₄^•-) and hydroxyl radicals (HO^•). The surface reactive Mn(IV) species are proposed to be responsible for the degradation of organic contaminants (e.g., phenol) and the formation of singlet oxygen (¹O₂), followed by the regeneration of the surface Mn(III) sites on Mn₂O₃. This study advances the fundamental understanding of the underlying mechanism involved in transition metal oxide-catalyzed PMS activation processes.