The effects of nonredox metal ions on the activation of peroxymonosulfate for organic pollutants degradation in aqueous solution with cobalt based catalysts: A new mechanism investigation.

Herein, a new peroxymonosulfate (PMS) activation system was proposed employing nonredox metal ions as Lewis acids (LA), which have been widely recognized to play important roles in many biological and chemical oxidations. With Co2+ ions as model catalysts, it was found that oxidizing power of PMS was enhanced after binding weak LA such as Ca2+ ions, leading to its easier reduction to active radicals and substantial enhancement of dye degradation. The promoting effect of Ca2+ was also observed with other cobalt catalysts including CoFe2O4 and Co3O4. The rate of PMS decomposition in Co2++LA/PMS system was correlated with Lewis acidity; while in the presence of strong LA including La3+ and Y3+, the dye degradation rate declined. The interactions of LA with PMS were characterized and the detailed mechanism was proposed. The present study provides the first example of the promoting effect of weak LA on PMS activation with cobalt based catalysts.

Lewis acids promoted organic pollutants degradation in aqueous solution with peroxymonosulfate and MnO2: New insights into the activation mechanism.

Nonredox metal ions have been widely recognized to be important in a wide range of biological and chemical oxidations as Lewis acids (LA). However, the role of LA in peroxymonosulfate (PMS) activation for wastewater treatment has not been considered until now. This study shows that oxidizing power of PMS can be promoted after binding nonredox metal ions such as Ca2+ as LA, leading to the easier reduction of the oxidant to radicals and substantial enhancement of dye degradation by employing manganese oxides OMS-2 as model catalysts. Increased with Lewis acidity of the metal ion, the rate of PMS decomposition enhanced linearly, while the dye degradation rate first increased and then declined due to the formation of a larger amount of dioxygen. The interactions between Ca2+ and PMS were further investigated by Raman, cyclic voltammetry and XPS; and the detailed mechanism of PMS activation was proposed. The performance of Ca2++OMS-2/PMS system under different conditions was also studied. The findings indicate the importance of LA in PMS activation reaction and their role must be considered in other transition metal oxides/PMS systems. It will be also helpful to design new and highly active catalysts for the reactions.

Tuning manganese (III) species in manganese oxide octahedral molecular sieve by interaction with carbon nanofibers for enhanced pollutant degradation in the presence of peroxymonosulfate.

The development of environmental-benign, efficient, and durable Mn based catalysts is of great importance for remediation of organic pollutants. In this study, a series of cryptomelane-type octahedral molecular sieve (OMS-2) and carbon nanofibers (CNFs) nanocomposites (OC) were synthesized by a facile refluxing approach under different conditions, and the catalytic activity was evaluated for Acid Orange 7 degradation under neutral conditions via peroxymonosulfate activation. It was revealed that the composites were more efficient than the pure OMS-2 and CNFs, due to the higher amounts of Mn(II) and Mn(III) species and the synergistic effects between OMS-2 and CNFs. The OC catalysts presented long-term stability without the leaching of Mn ions during seven consecutive cycles. Radical scavenger and EPR experiments indicated that the low valent Mn species in the composites were oxidized by PMS to produce sulfate radicals to degrade dyes. However, the structure and performance of OC were influenced significantly by CNFs dosage and refluxing time. Under a high CNFs ratio or a long refluxing time, OMS-2 was damaged and lost its catalytic activity completely. This study provides important implications for turning of the valence of Mn species in manganese oxides, and widens the practical applications of the manganese oxides/ carbon materials in wastewater treatment.

Room-temperature synthesis of OMS-2 hybrids as highly efficient catalysts for pollutant degradation via peroxymonosulfate activation.

Due to low cost and low toxicity, manganese oxides have been extensively explored to reduce organic pollutants in wastewater via peroxymonosulfate (PMS) activation; but the development of manganese-based catalysts with facile synthesis process at low temperatures and high efficiency is still of significant practical interest. In this paper, a simple room temperature method has been successfully developed to synthesize cryptomelane-type manganese octahedral molecular sieves (OMS-2) from KMnO4 and MnSO4 in the presence of carbon nanotubes (CNTs). The redox reaction between amorphous manganese oxide and CNTs results the decrease of manganese valences and the formation of OMS-2 phase at a low temperature of 25 °C. The changes of synthesis time, temperature and CNTs dosage altered the characteristics of the prepared materials. Compared with CNTs, OMS-2 and other manganese oxides hybrids, the synthesized catalysts demonstrated a remarkable efficiency for PMS activation to degrade organic dyes and a superior reusability during ten successive cycles. Sulfate radicals were formed as the active species in the system from the oxidation of low valent Mn species by PMS. This study not only provides a simple method to synthesize OMS-2 at room temperature, but also improves the understanding of PMS activation on manganese-based catalysts for pollutants degradation.

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.

Photocatalytic degradation of organic pollutants in wastewater with g-C3N4/sulfite system under visible light irradiation.

To develop low cost and high efficient sulfate radical (SO4-) based advanced oxidation processes (AOPs) for rapid remediation of contaminated waters is of great interest. In this study, a green and novel SO4- based AOPs, in situ visible light activation of sulfite by graphitic carbon nitride (g-C3N4), for the degradation of organic pollutants is reported. The g-C3N4+HSO3- + Vis system could achieve remarkably enhanced degradation of organic pollutants such as organic dyes and phenol in aqueous solution. The excellent reusability of the metal free catalyst was also observed during ten successive cycles. The efficiency of the system was dependent on the reaction conditions, which first increased and then decreased with the increase of HSO3- concentration and initial solution pH. The addition of HCO3- stimulated the pollutant degradation, but other water matrix components such as Cl- and humic acid showed nearly no influence on the reaction. The mechanism investigations suggested that sulfite is oxidized in the system to sulfite radicals, which then react with dioxygen and superoxide radicals to form SO5- radicals and HSO5- respectively. SO5- radicals can be also reduced by sulfite or photoelectron to HSO5-. SO4- radicals were then produced from HSO5- reduction by photoelectron, and contributed to dye degradation in the system together with superoxide radicals. This study provides a novel new approach for efficient degradation of organic degradation via sulfite activation.