Aihua Xu

Metal-free activation of peroxymonosulfate by g-C3N4 under visible light irradiation for the degradation of organic dyes

Semiconducting carbon nitride materials (g-C3N4) were investigated as metal-free catalysts for the activation of peroxymonosulfate (PMS) under visible light irradiation to degrade organic dyes in aqueous solution. X-ray diffraction, scanning electron microscopy, N2 adsorption/desorption isotherms, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to characterize the properties of the material. The effect of several parameters including the concentration of catalyst, PMS and organic dye, and initial solution pH on its catalytic activity was also investigated. It was found that the obtained g-C3N4 can effectively activate PMS under visible light irradiation to generate strong sulfate radicals which was highly active for Acid Orange 7 (AO7) and other organic dyes degradation. The catalyst also presented a long-term stability during multiple runs. Based on intermediate detections, the degradation pathway of AO7 in the g-C3N4/PMS/Vis system was proposed. This study demonstrated a promising approach for the activation of green oxidant, PMS, by the newly-developed polymer photocatalysts for environmental remediation and oxidation catalysis.

Catalytic degradation of Acid Orange 7 by manganese oxide octahedral molecular sieves with peroxymonosulfate under visible light irradiation.

In this paper, the photodegradation of Acid Orange 7 (AO7) in aqueous solutions with peroxymonosulfate (PMS) was studied with manganese oxide octahedral molecular sieves (OMS-2) as the catalyst. The activities of different systems including OMS-2 under visible light irradiation (OMS-2/Vis), OMS-2/PMS and OMS-2/PMS/Vis were evaluated. It was found that the efficiency of OMS-2/PMS was much higher than that of OMS-2/Vis and could be further enhanced by visible light irradiation. The catalyst also exhibited stable performance for multiple runs. Results from ESR and XPS analyses suggested that the highly catalytic activity of the OMS-2/PMS/Vis system possible involved the activation of PMS to sulfate radicals meditated by the redox pair of Mn(IV)/Mn(III) and Mn(III)/Mn(II), while in the OMS-2/PMS system, only the redox reaction between Mn(IV)/Mn(III) occurred. Several operational parameters, such as dye concentration, catalyst load, PMS concentration and solution pH, affected the degradation of AO7.

Efficient degradation of organic pollutants in aqueous solution with bicarbonate-activated hydrogen peroxide.

Bicarbonate anion is an efficient activator for hydrogen peroxide to generate many active oxygen species including peroxymonocarbonate (HCO(4)(-)), superoxide ion (O(2)(-)) and singlet oxygen ((1)O(2)). This study aims to understand the oxidative degradation of organic pollutants including methyl blue, methyl orange, rhodamine B, and 4-chlorophenol, with H(2)O(2) activated by sodium bicarbonate at room temperature. The obtained results indicate that such a method is apparently efficient in versatile pollutant degradation. Compared with using H(2)O(2) alone under similar pH conditions, the degradation rates of the pollutants were greatly enhanced through adding NaHCO(3). Through LC-MS, FT-IR and the TOC analysis, the degradation of methylene blue was revealed to proceed by the transformation of dimethylamino group in methylene blue to methylamino, aldehyde and nitro group, and the opening of phenyl ring into small molecular compounds and CO(2). The studies using the (1)O(2) scavenger sodium azide and the O(2)(-) indicator nitro blue tetrazolium suggest that the active O(2)(-) intermediate, generated from HCO(4)(-) decomposition, rather than (1)O(2) was involved in the pollutant degradation.

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.

Decolorization of Acid Orange II dye by peroxymonosulfate activated with magnetic Fe3O4@C/Co nanocomposites

New metallic cobalt loaded magnetic nanocomposites (Fe3O4@C/Co) were prepared by calcination and in situ reduction of the Co2+-impregnated magnetic carbon nanoparticles of Fe3O4@C at 900 °C under a nitrogen atmosphere. Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), vibrating sample magnetometry, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the properties of the composites and the loading of metallic Co0 in the material was clarified. The catalytic properties of the nanocomposites Fe3O4@C/Co in activating peroxymonosulfate (PMS) for decolorization of Acid Orange II (AO II) dye in aqueous solution were investigated. Fe3O4@C/Co nanocomposites exhibited high activity in PMS activation for AO II decolorization. Complete decolorization of AO II solution could be achieved in 20 min within near neutral pH range (pH 6.4–8.5), while a complete decolorization occurred in 40 min for the as-prepared solution without pH adjustment (pH 4.05). The effects of several parameters including pH, catalyst load, PMS concentration and reaction temperature on the catalytic activity were also investigated. Sulfate free radicals activated from PMS were proposed to be the dominant active species in the “Fe3O4@C/Co + PMS” system for AO II decolorization. The catalytic and decolorization mechanism was suggested. The catalyst Fe3O4@C/Co could be recycled easily by a magnet with good reusability. This study provides a promising method for the activation of “green” oxidant, PMS, by the new magnetic nanocomposites for environmental remediation and oxidation catalysis.

Decolorization of Dye Pollutions by Manganese Complexes with Rigid Cross-Bridged Cyclam Ligands and Its Mechanistic Investigations

Exploring pH-insuspensible catalysts for pollutant treatments is still a challenge because of the pH-dependent catalyst stability. A recently developed manganese complex, Mn(Me2EBC)Cl2, (Me2EBC: 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) was investigated to decolorize dyes in the pH range of 1-13 using H2O2 as the oxidant, and its stability in service was examined by periodically adding fresh dye and oxidant. The results display that this manganese complex is capable of decoloring dye in both acid and base and keeping activity for a long service period. In addition, its modified analogue, Mn(Et2EBC)Cl2, can perform efficient decolorization in the whole pH range from pH 1 to 13. Combinatorial methods including stoichiometric treatments of dye with freshly synthetic manganese(IV) complex, utilization of radical scavenger, and intermediate analysis by UV-visible spectrophotometry were applied for its mechanistic studies, and a manganese(IV) hydroperoxide intermediate has been suspected to serve as the key active species in decolorization.

The facile synthesis of a magnetic OMS-2 catalyst for decomposition of organic dyes in aqueous solution with peroxymonosulfate

A magnetically recyclable nanocomposite, manganese oxide octahedral molecular sieve (OMS-2) coated Fe3O4 nanoparticles, was prepared using a facile solvent-free process with manganese salts and Fe3O4 at a low temperature. XRD, SEM, TEM, FT-IR, XPS and N2 adsorption–desorption isotherm analysis showed that in the core–shell nanocomposite, OMS-2 was homogeneously distributed on the Fe3O4 surface with similar structures to that of pure OMS-2 but a much higher surface area of 251 m2 g−1. The catalyst exhibited excellent activity toward the degradation of Acid Orange 7 (AO7), Reactive Brilliant Blue KN-R, Reactive Brilliant Red X-3B, methylene blue, methyl orange, and rhodamine B in the presence of peroxymonosulfate (PMS), with nearly complete decolorization within 15 min. The catalyst could also be easily recovered with the assistance of an external magnetic field, and could be reused for ten successive cycles without any change of structure. The loss of Mn ions in solution was lower than 0.05 mg L−1 and no Fe ions were detected. The effect of several parameters, including the OMS-2 content in the nanocomposites, the concentration of AO7, PMS and the catalyst, and solution pH, on the degradation efficiency was also discussed.

Efficient production of glucose by microwave-assisted acid hydrolysis of cellulose hydrogel.

To improve the production of glucose from cellulose, a simple and effective route was developed. This process uses a combination of a step of cellulose dissolution in aqueous NaOH/urea solution and then regeneration with water, followed by an acid hydrolysis step under microwave irradiation. The method is effective to obtain glucose from α-cellulose, microcrystalline cellulose, filter paper, ramie fiber and absorbent cotton. Increased with the acid concentration the glucose yield from hydrogel hydrolysis increased from 0.42% to 44.6% at 160 °C for 10 min. Moreover, the ozone treatment of cellulose in NaOH/urea solution before regeneration significantly enhanced the hydrolysis efficiency with a glucose yield of 59.1%. It is believed that the chains in cellulose hydrogel are relatively free approached, making that the acids easily access the β-glycosidic bonds.

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

A graphitic carbon nitride-containing MCM-41 catalyst (g-C3N4/MCM-41) was prepared through an in situ thermal approach and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, N2 adsorption–desorption and X-ray photoelectron spectroscopy. It was found that Acid Orange 7 (AO7) and other five organic dyes can be completely degraded by the g-C3N4/MCM-41 catalyst within 30 min in the presence of peroxymonosulfate (PMS), while g-C3N4 + PMS and MCM-41 + PMS show nearly no activity. A non-radical pathway accompanied by radical generation in PMS activation and AO7 oxidation is proposed. The oxidized carbon species N–C–O on g-C3N4 with strong electron density plays a key role in the reaction, while the large surface area of the catalyst can also efficiently enhance the accessibility of catalytic active sites. Though the reusability of the g-C3N4/MCM-41 catalyst is not very good as that of metal oxide catalysts, its activity can be recovered by KOH treatment. The study can broaden the application of g-C3N4 in the area of wastewater treatment.

Synthesis of OMS-2/graphite nanocomposites with enhanced activity for pollutants degradation in the presence of peroxymonosulfate

The octahedral molecular sieve manganese oxide (OMS-2) and graphite (Gt) composites (OMS-2/Gt) were prepared by a facile refluxing approach. The structure and morphology of the obtained materials were systematically investigated. The results reveal that OMS-2 nanofibers are uniformly dispersed on the surface of Gt, and the ratio of Mn3+ species in OMS-2 increases lineally with the increase of Gt dosage. The catalytic activity of OMS-2/Gt was evaluated by the degradation of Acid Orange 7 (AO7) in the presence of peroxymonosulfate (PMS). It is found that the OMS-2/Gt composites show enhanced catalytic performance with the degradation rate lineally correlated with the content of OMS-2 in the catalyst, and even with the ratio of Mn3+/Mn4+. XPS and radical scavenger experiments further indicated that the oxidation of Mn3+ by PMS system occurs in the system with the formation of hydroxyl radicals contributed to the dye degradation. The catalysts also exhibit good stability and reusability during consecutive cycles. Thus, the environmental friendly OMS-2/Gt composites with low cost, facile synthesis process and high efficiency are very promising catalysts for PMS activation and pollutants degradation.