Effectiveness And Mechanism Of The Oxidation Of Phenolic Compounds By Hydrogen Peroxide, Peroxymonosulfate And Permanganate Catalyzed With Iron And Manganese

There has been increasing concern about the widespread occurrence of phenolic compounds in the aquatic environment. Hydrogen peroxide, peroxymonosulfate (PMS), and permanganate are mostly available green oxidant but don't show great potential in treating phenolic pollutants due to their relatively weak oxidation ability. In this regard, the development of their related catalytic technologies, which can enhance these oxidants to decompose to highly reactive oxidant, has received great interests. In this work, the kinetics and mechanism of the oxidative degradation of phenolic compounds by hydrogen peroxide, peroxymonosulfate, and permanganate catalyzed with transition metals iron and manganese were investigated, with a focus on assessing the potential of these catalytic technologies for the oxidative removal of phenolics during water/wastewater treatment.The oxidation of phenolic compounds by traditional Fenton reagent (Fe(II,III)/H₂O₂) displayed autocatalysis, suggesting the catalytic role of in-situ formed quinone intermediates. Further experiments indicated that quinones can act as electron shuttles to promote the conversion from ferric ions to ferrous ions, and consequently enhanced the oxidation rates of phenolics. The phenolics solution during reaction underwent a fast color change from colorless to dark brown and later to a light brown. The mixture of single p-benzoquinone (yellow) and hydroquinone (colorless) solutions fit the color observed in the reacting mixture (brown); these highly colored species were well-known as charge-transfer complexes called quinhydrones. Similarly, activated carbons were also noted to have the ability to promote the conversion from ferric ions to ferrous ions and consequently enhance the degradation of phenolic compounds by Fenton reaction. This was reasonably explained by the contribution of surface quinones functional groups on activated carbons.Using sulfoxides as probe compounds, it was shown that ferryl (Fe(IV)) species could react with sulfoxides through an oxygen-atom-transfer step producing corresponding sulfones, which markedly differes from their·OH-involved products. Hydroxyl radicals rather than ferryl ions were identified as reactive intermediates in the Fenton reaction at acidic and neutral pH. The addition of several ligands (EDTA, oxalate, and citrate) to Fenton reaction did not alter the oxidant nature over the wide pH range of 3-9. However, at neutral and alkaline pH conditions, not only hydroxyl radicals but also ferryl ions were detected in Fenton reaction in the presence of NTA. The in-situ formed Fenton reagent during the corrosion of zero-valent iron by oxygen was capable of oxidizing various organic and inorganic compounds. The background matrice of real waters and several selected ligands (e.g., EDTA, NTA, oxalate, and citrate) could significantly enhance the oxidation capacity by limiting iron precipitation and/or by accelerating the rates of key reactions including ferrous ions oxidation by oxygen and hydrogen peroxide. Addition of scavengers for hydroxyl radicals and/or ferryl ions completely inhibited the oxidation of arsenite at acidic pH but had negligible effects at circumneutral pH. This result suggested that the oxidation of As(III) at circumneutal pH mainly occurred on iron surface rather than in bulk solution.Manganese oxides (MnO₂) possessed the ability to catalyze the oxidative degradation of phenolics by peroxymonosulfate via a simiar mechanism to that of permanganate; this was, after formation of surface precursor complexes between phenolics and the surface-bound MnIV, they were quickly oxidized by peroxymonosulfate and permanganate with a much higher rate than their solution counterparts. The oxidation kinetics of phenolic compounds by peroxymonosulfate catalyzed with manganese(II) complexes were noted to display autocatalysis. Stable manganese(III) complexes identified in the reaction mixture by on-line UV-vis scanning and capillary electrophoresis were assumed to be the real catalyst. This was further suppoted by the observation that the Mn(III)L/PMS system showed much higher reactity than the Mn(II)L/PMS system, and the autocatalysis disappeared. The selective oxidation exhibited by Mn(II,III)L/PMS system with scavengers suggested that the reactive species was probably Mn(V) but not hydroxyl or sulfate radical.The background matrices in real waters and several selected ligands were observed to significantly promote permanganate oxidation, which was explained by the contribution of ligand-stabilized manganese intermediates in situ-formed upon permanganate reduction. Stable Mn(III) complexes were identied by the on-line UV scaning and capillary electrophoresis. These species, which were prepared ex-situ ( 4Mn(II)L+Mn(VII)+L→5Mn(III)L), could readily oxidize phenolics but exert no reactivity toward CBZ and RMSO. This was consistent with the finding that selected ligands could exert oxidation enhancement for phenolics but negligible influence for CBZ and RMSO. Moreover, the combination of the one-electron reduction of Mn(III) ( Mn(III) + e ?→Mn(II)) and the Mn(VII)/Mn(II) reaction in excess ligands ( Mn(VII) + 4Mn(II) ?L?igan?ds→5Mn(III)) suggested a catalytic role of the Mn(III)/Mn(II) pair in permanganate oxidation of some phenolics in the presence of ligands. The kinetic model RCT which involved the contribution of in-situ formed Mn(III) species underpredicted the oxidation kinetics of phenolics by permanganate in the presence of ligands, indicating other unidentified highly reactive manganese intermediates (e.g., Mn(V)) propable participate in the oxidation reaction.