| Catalytic ozonation is very effective in removal of persistent organic compounds in water and wastewater. It has been reported that manganese oxides is active in ozone decomposition, but its application in catalytic ozonation was not intensively studied. In this paper, we synthesized a series of manganese oxides with different polymorphs, valences and composition, and applied in catalytic ozonation of phenols in wastewater. The different reactive species in weak acid and basic solutions were revealed, and the influence of substituents to the phenols degradation rate and reactive species was studied. The interaction to each other in the mixed solution during phenols degradation existed. From accurate analysis of intermediate products, we proposed possible reaction pathways in mineralization of phenol mixtures.The thesis was to assess the suitability of selected catalytic ozonation systems for the depuration of phenolic mixture, understanding reaction pathway, influence electron donating group (EDG) and electron with drawing group (EWG), reactive species, in order to achieve extensive removal and high mineralisation rates.Firstly, all α, β,γ,δ, ε, and λ polymorphs of manganese dioxides were synthesized and used in catalytic ozonation of p-nitrophenol (4-NP). α-MnO2 was considered to be more efficient in catalytic ozonation because strong interaction between ozone and α-MnO2 was observed and confirmed as a critical step for the catalysis reaction. The hydroxyl groups and chemisorbed water on the catalyst surface were proposed as active sites in generating active oxygen species while the Lewis acid sites were confirmed as the reactive center for catalytic ozonation in the aqueous phase. Scavenger’s tert- butanol (t-BA) and p-benzoquinone (p-BQ) and sodium azide (NaN3) were used to evaluate the contributions of different radicals. The result showed that hydroxyl radical was not responsible for 4-NP removal in ozonation and catalytic ozonation, while superoxide radical showed great contribution.Two series of α-MnO2 materials were prepared via a similar method with surfactant cetyltrimethylammonium bromide (CTAB) and sodium dodecyl benzene sulfate (SDBS). The surfactants worked both as shape directing agent and reduction initiator. The α-MnO2 synthesized with 0.2 M of CTAB showed a mesoporous structure with high surface area of 387.7 m2/g. It is very active in catalytic ozonation of p-nitrophenol (4-NP) and TOC removal, with higher catalytic stability than commercial α-MnO2. Using t-BA and p-BQ as scavengers of different oxidative species, we also found that superoxide radicals, rather than hydroxyl radicals, were mainly responsible for the degradation of 4-NP.A series of shape controlled manganese oxides were prepared and applied in catalytic ozonation of phenol (Ph),p-cresol (Ph-CH3) and p-chlorophenol (Ph-Cl). MnO2 was found to be more active than Mn2O3 and Mn3O4 for its cubic structure and larger surface area. The degradation order was Ph-CH3> Ph-Cl> Ph in the solution with single component, and this became Ph-Cl> Ph-CH3> Ph in the mixed solution, indicating that both electron donating group (EDG) and electron withdrawing group (EWG) benefit phenol degradation. The co-existed substituted phenol had different effects to the degradation of the other in ozonation and catalytic ozonation of the mixed solution, but the influence to each other was universally opposite. Quenching experiments with different scavengers were conducted to reveal that superoxide radical, singlet oxygen and ozone molecular, rather than hydroxyl radical, were directly responsible for phenols degradation. The contribution of ozone and singlet oxygen on phenol degradation follow the order Ph-CH3> Ph> Ph-Cl, indicating an electrophilic attacking reaction, while the trend is opposite on superoxide radical oxidation possibly.Mesoporous Fe3O4/MnO2 composite was fabricated by co-precipitation method, and used in degradation of Ph-CH3 and Ph-Cl mixture. The synergetic catalytic effect of Fe3O4 and MnO2 was observed in their composite material, which is attributed to mesoporous morphology and composite material. ATR-FTIR results indicated that ozone competitively adsorbed on the surface of Fe3O4/MnO2 composite with water molecule and converted into hydroxyl radical, meanwhile the catalyst surface underwent oxidation and reduction as demonstrated by CV and others experimental data. We also find the acidic, basic and neutral medium affects phenolic compounds degradation, the order of phenolic compounds in mixture changed from Ph-CH3> Ph-Cl to Ph-Cl> Ph-CH3, when the reaction medium was changed from basic to neutral and acidic. GCMS results indicated the evolution of intermediates and the mineralization pathway from the original phenolic mixture to CO2 and H2O. ESR and quenching experiments with different scavengers were conducted to reveal that hydroxyl radical and ozone molecule were responsible for phenolic mixture degradation at pH 9. |
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