Facile fabrication of CoAl-LDH nanosheets for efficient rhodamine B degradation via peroxymonosulfate activation

Layered double hydroxides (LDHs) have been extensively investigated as promising peroxymonosulfate (PMS) activators for the degradation of organic pollutants. However, bulk LDHs synthesized using conventional methods possess a closely stacked layered structure, which seriously blocks active sites and yields low intrinsic activity. In this study, we exfoliated bulk CoAl-LDHs to fabricate CoAl-LDH nanosheets by alkali-etching and Ostwald ripening via a simple hydrothermal process in a KOH solution. The exfoliated LDHs possessed the typical nanosheet structure with more exposed active sites for PMS activation, and hence, boosted the degradation of the pollutants. CoAl-1 exhibited an outstanding catalytic performance as the PMS activator for rhodamine B (RhB) degradation with the apparent rate constant of 0.1687 min-1, which was about 3.63 and 5.02 times higher than that of commercial nano-Co3O4 and bulk CoAl-LDH, respectively. The maximum RhB degradation of 93.1% was achieved at the optimal reaction conditions: catalyst dose 0.1 g L-1, PMS concentration 0.3 mM, pH 7, and temperature 298 K. Further analysis of RhB degradation mechanism illustrated that singlet oxygen (1O2) dominated RhB degradation in the CoAl-1/PMS system, while ˙OH, ˙O2-, and ˙SO4- may mainly serve as the intermediates for the generation of 1O2 and were indirectly involved in the degradation. This study provides a promising strategy for developing two-dimensional LDH nanosheets for wastewater remediation.

One-step pyrolysis for preparation of sulfur-doped biochar loaded with iron nanoparticles as an effective peroxymonosulfate activator for RhB degradation

In this study, sulfur-doped biochar loaded with iron nanoparticles (Fe/S-BC) was easily prepared by a one-pot pyrolysis method using anhydrous FeCl3, Na2S2O3·5H2O, and cherry stone powder as raw materials and...

Synthesis of polyimide-modified carbon nanotubes as catalyst for organic pollutant degradation via production of singlet oxygen with peroxymonosulfate without light irradiation

Polyimide-modified carbon nanotubes (PI/CNTs) were synthesized via a solvent-free thermal method and used as a metal-free catalyst to activate peroxymonosulfate for organic contaminant degradation without light irradiation. The characterization results suggested that PI was loaded onto the surface of CNTs. The catalytic ability of the PI/CNTs was strongly correlated with the content of PI in the catalysts. The PI/CNTs (22% of PI) showed the highest catalytic efficiency for organic pollutant degradation at room temperature. The degradation efficiency of acid orange 7 (AO7) dye was significantly enhanced to 98.9% within 15 min, compared to the efficiency of 2.2% exhibited by pure PI. The radical quenching tests and electron paramagnetic resonance spectrometry proved that singlet oxygen, instead of hydroxyl radicals or sulfate radicals, played a dominant role during the catalytic oxidation of AO7. The influences of operation parameters including temperature and catalyst amount were investigated. The PI/CNTs metal-free catalyst exhibited high catalytic activity under a broad range of pH values. The recycling study of four repeated reactions demonstrated good stability of the PI/CNTs. This work provided a promising metal-free catalyst for degradation of organic pollutants in aqueous solutions, contributing to the development of green materials for sustainable remediation.

Mn-based catalysts for sulfate radical-based advanced oxidation processes: A review

Sulfate radical-based advanced oxidation processes (AOPs) have drawn increasing attention during the past two decades, and Mn-based materials have been proven to be effective catalysts for activating peroxymonosulfate (PMS) and peroxydisulfate (PDS) to degrade many contaminants. This article presents a comprehensive review of various Mn-based materials to activate PMS and PDS. The activation mechanisms of different Mn-based catalysts (i.e., Mn oxides MnO_x, MnO_x hybrids, and MnO_x‑carbonaceous material composites) were first summarized and discussed in detail. Besides the commonly reported free radicals (SO₄^-• and ^•OH), non-radical mechanisms such as singlet oxygen and direct electron transfer have also been discovered for selected materials. The effects of pH, inorganic ions, natural organic matter (NOM), dissolved oxygen content, temperature, and the crystallinity of the materials on the catalytic reactivity were also discussed. Then, important instrumentations and technologies employed to characterize Mn-based materials and to understand the reaction mechanisms were concisely summarized. Three common overlooks in the experimental designs for examining the PMS/PDS-MnO_x systems were also discussed. Finally, future research directions were suggested to further improve the technology and to provide a guidance to develop cost-effective Mn-based materials to activate PMS/PDS.

Sulfate radical-mediated degradation of phenol and methylene blue by manganese oxide octahedral molecular sieve (OMS-2) activation of peroxymonosulfate

Activation of peroxymonosulfate (PMS) has been concentrated on degrading refractory organic pollutants owing to the generation of sulfate radical ([Formula: see text]) with high standard redox potential. In this study, manganese oxide octahedral molecular sieve (OMS-2) with cryptomelane type was synthesized by a new hydrothermal method to activate PMS for the degradation of phenol and methylene blue (MB) in water. The as-prepared composites were fully characterized, and the effects of PMS dosage, OMS-2 dosage, initial pollutant concentration, pH, and chloride on the degradation of phenol were elaborately investigated. Moreover, the phenol degradation was evaluated through the variations of total organic carbon (TOC) and three-dimensional excitation emission matrix (3D-EEM), and reaction intermediates were also investigated. Both electron spin resonance (ESR) spectra and comparative experiments suggested [Formula: see text] and hydroxyl radical (HO^•) took part in the phenol degradation and [Formula: see text] was more significant than HO^•. The fine degradation efficiency of phenol in different water source, as well as the stability after continuous use, indicated the possible application of PMS/OMS-2 in real wastewater treatment.

Catalytic degradation of diclofenac from aqueous solutions using peroxymonosulfate activated by magnetic MWCNTs-CoFe3O4 nanoparticles

CoFe₃O₄ nanoparticles supported on multi-walled carbon nanotubes (MWCNTs-CoFe₃O₄) were synthesized by the co-precipitation method as a novel catalyst for degradation of diclofenac (DCF). The comparative experiments indicated that MWCNTs-CoFe₃O₄ has a better catalytic activity in degradation of DCF and activation of peroxymonosulfate (PMS) compared to other catalytic systems. This can be attributed to the interaction of MWCNTs with CoFe₃O₄ in accelerating the absorption process and activating the PMS (E_a = 22.93 kJ mol⁻¹). The removal efficiencies of DCF and total organic carbon (TOC) were 99.04% and 50.11%, under optimum conditions, e.g., pH of 7, PMS dosage of 4 mM, DCF concentration of 30 mg L⁻¹, catalyst dosage of 500 mg L⁻¹, and reaction time of 120 min. The oxidation of DCF was fitted by the pseudo-first-order kinetic model and the constant rate was increased by increasing the pH, temperature, dosage of PMS and catalyst. The production of reactive species was studied using scavengers such as TBA and ethanol and the results showed that sulfate radical is the reactive species responsible for the degradation of DCF. The MWCNTs-CoFe₃O₄ catalyst showed high stability and reusability based on five successful repeated reactions, X-ray diffraction and energy dispersive X-ray spectroscopy analysis. Based on the intermediates detected by gas chromatography-mass spectrometry (GC-MS), the possible pathways for DCF catalytic oxidation were proposed. The results explained that the PMS/MWCNTs-CoFe₃O₄ system is a promising method for treating DCF solution due to high efficiency, good reusability of catalyst and greater PMS activation.

The MWCNTs-CoFe₃O₄ as a novel catalyst showed high catalytic activity in activation of proxymonosulfate for degradation of diclofenac.

Controlled growth of γ-MnO2 nanoflakes on OMS-2 for efficient decomposition of organic dyes in aqueous solution via peroxymonosulfate activation

The development of green and efficient catalysts for peroxymonosulfate (PMS) activation and abatement of organic pollutants in wastewater is of significant practical interest. In this paper, the three-dimensional mixed manganese oxides of OMS-2 and γ-MnO₂ were fabricated through a simple refluxing method from KMnO₄ and MnSO₄. It was found that growth of γ-MnO₂ nanoflakes on OMS-2 can be controlled by the concentration of MnSO₄. The catalysts not only have many excellent structural properties such as interconnected network and highly exposed active sites, but also show the high ratio of low valent manganese species. In particular, the catalysts exhibited much higher efficiency for Acid Orange 7 degradation in the presence of PMS than pure OMS-2 or γ-MnO₂. The oxidation of Mn(III) species by PMS occurs in the system with the formation of sulfate and hydroxyl radicals contributed to the dye degradation. Moreover, the catalysts showed good stability and reusability during four consecutive cycles. Thus, the environmental friendly mixed manganese oxides of γ-MnO₂ and OMS-2 with low cost, facile synthesis process and high efficiency are very promising catalysts for PMS activation and pollutants degradation.

Heterogeneous oxidative synthesis of quinazolines over OMS-2 under ligand-free conditions

OMS-2 is employed to synthesize heterocycles through selective oxidation without the help of ligands.

Application of magnetic OMS-2 in sequencing batch reactor for treating dye wastewater as a modulator of microbial community

The potential and mechanism of synthesized magnetic octahedral molecular sieve (Fe₃O₄@OMS-2) nanoparticles in enhancing the aerobic microbial ability of sequencing batch reactor (SBR) for treating dye wastewater have been revealed in this study. The addition of Fe₃O₄@OMS-2 of 0.25g/L enhanced the decolorization of SBRs with an operation cycle of 24h by more than 20%. The 16S rRNA gene high-throughput sequencing indicated Fe₃O₄@OMS-2 increased the microbial richness and diversity of SBRs, and more importantly, promoted the potential dye-degrading bacteria. After a series of enriching and screening, four bacterial strains with the considerable decolorizing ability were isolated from SBRs, designating Alcaligenes faecalis FP-G1, Bacillus aryabhattai FP-F1, Escherichia fergusonii FP-D1 and Rhodococcus ruber FP-E1, respectively. The growth and decolorization of these pure strains were promoted in the presence of Fe₃O₄@OMS-2, which agrees with the result of high-throughput sequencing. Monitoring dissolved Fe/Mn ions and investigating the change of oxidation states of Fe/Mn species discovered OMS-2 composition played the critical role in modulating the microbial community. The significant enhancement of Mn-oxidizing/-reducing bacteria suggested microbial Mn redox may be the key action mechanism of Fe₃O₄@OMS-2, which can provide numerous benefits for the microbial community and decolorization of SBRs.

The effects of manganese oxide octahedral molecular sieve chitosan microspheres on sludge bacterial community structures during sewage biological treatment

This study examines the effects of manganese oxide octahedral molecular sieve chitosan microspheres (Fe₃O₄@OMS-2@CTS) on anaerobic and aerobic microbial communities during sewage biological treatment. The addition of Fe₃O₄@OMS-2@CTS (0.25 g/L) resulted in enhanced levels of operational performance for decolourization dye X-3B. However, degradation dye X-3B inhibition in the presence of Fe₃O₄@OMS-2@CTS was recorded as greater than or equal to 1.00 g/L. Illumina MiSeq high throughput sequencing of the 16 S rRNA gene showed that 108 genera were observed during the anaerobic process, while only 71 genera were observed during the aerobic process. The largest genera (Aequorivita) decreased from 21.14% to 12.65% and the Pseudomonas genera increased from 10.57% to 12.96% according to the abundance in the presence of 0.25 g/L Fe₃O₄@OMS-2@CTS during the anaerobic process. The largest Gemmatimonas genera decreased from 21.46% to 11.68% and the Isosphaerae genera increased from 5.8% to 11.98% according to the abundance in the presence of 0.25 g/L Fe₃O₄@OMS-2@CTS during the aerobic process. Moreover, the X-ray photoelectron spectroscopy results show that the valence states of Mn and Fe in Fe₃O₄@OMS-2@CTS changed during sewage biological treatment.

Activation of peroxymonosulfate by graphitic carbon nitride loaded on activated carbon for organic pollutants degradation

Graphitic carbon nitride supported on activated carbon (g-C3N4/AC) was prepared through an in situ thermal approach and used as a metal free catalyst for pollutants degradation in the presence of peroxymonosulfate (PMS) without light irradiation. It was found that g-C3N4 was highly dispersed on the surface of AC with the increase of surface area and the exposition of more edges and defects. The much easier oxidation of C species in g-C3N4 to CO was also observed from XPS spectra. Acid Orange 7 (AO7) and other organic pollutants could be completely degraded by the g-C3N4/AC catalyst within 20min with PMS, while g-C3N4+PMS and AC+PMS showed no significant activity for the reaction. The performance of the catalyst was significantly influenced by the amount of g-C3N4 loaded on AC; but was nearly not affected by the initial solution pH and reaction temperature. In addition, the catalysts presented good stability. A nonradical mechanism accompanied by radical generation (HO and SO4(-)) in AO7 oxidation was proposed in the system. The CO groups play a key role in the process; while the exposure of more N-(C)3 group can further increase its electron density and basicity. This study can contribute to the development of green materials for sustainable remediation of aqueous organic pollutants.

Catalytic performance of supported g-C3N4 on MCM-41 in organic dye degradation with peroxymonosulfate

The supported graphitic carbon nitride on MCM-41 was prepared through an in situ thermal approach and was successfully used as an efficient catalyst for dyes degradation with peroxymonosulfate.

Polyimides as metal-free catalysts for organic dye degradation in the presence peroxymonosulfate under visible light irradiation

Polyimide was demonstrated as a stable metal-free photocatalyst to activate peroxymonosulfate with visible light for the degradation of organic dyes.