Enhancement Of Organic Pollutant Degradation By Peroxides And The Reaction Mechanism

Peroxides have been widely employed as bulk oxidants for degrading organic compounds in soil and water. However, these peroxides inherently show weak reactivity toward organic compound and thus require activation to generate strong oxidizing species such as ?OH and SO4?-, which can degrade organic pollutants fast and effectively. Main activation processes include energy activation(such as light, heat, electricity, microwave, ultrasound, etc.), catalyzer activation by transition metal ions and their compounds, activation by electron transfer from light-excited photosensitive material, and anion activation.In this study, based on the reported activation methods, various peroxides were employed to investigate that methylene blue decolorization by oxygen-containing anions activated hydrogen peroxide, pentachlorophenol degradation in silica sand slurries by different Fe(II) sources activated peroxides, phenol degradation by near-UV light and phosphotungstic acid catalyzed peroxides, and the reoxidation of reduced phosphotungstic acid in photocatalytic reaction by peroxides. Removal efficiency of organic pollutants, reaction kinetics, influencing factors and reaction mechanism were systematically studied. The main results are as the following:1. Efficient decolorization of methylene blue solution was achieved by applying solid peroxides(sodium percarbonate, sodium perborate and calcium peroxide) with “build-in” oxygen-containing anions. Phosphate significantly enhanced the rates for methylene blue decolorization and active oxidant decomposition in the treatments with hydrogen peroxide, urea hydrogen peroxide, sodium percarbonate or sodium perborate under moderately alkaline condition. The strong correlation between methylene blue decolorization and active oxidant decomposition implied a shared pathway. Alkaline conditions proved to be essential for phosphate to activate hydrogen peroxide to decolorize methylene blue. The ability for oxygen-containing anions to activate hydrogen peroxide to decolorize methylene blue under moderate alkaline condition followed the order, HPO42- > HCO3-/CO32- > B(OH)4- > OH-.2. Four types of solid peroxides(calcium peroxide, sodium perborate, sodium percarbonate, urea hydrogen peroxide) and H2O2, and two iron sources(Fe3(PO4)2 and Fe SO4) were used as Fenton-like reagents to degrade pentachlorophenol(PCP) in silica sand slurries. Compared to traditional Fenton reagents, solid Fenton-like reagents could significantly extend active components’ life time from less than 24 hours to more than 20 days, and thus possess greater potential for soil contamination remediation. Fe3(PO4)2 supplied Fe(II) as a steady source to activate peroxides to degrade PCP continuously. p H buffer was required for a better decontamination performance when using alkaline inducing solid peroxides(calcium peroxide, sodium perborate, sodium percarbonate) as Fenton-like reagents. Urea hydrogen peroxide might be an alternative for H2O2.3. Hydrogen peroxide, peracetic acid and sodium persulfate under near-UV light irradiation were able to degrade phenol, with the degradation rate following the order of sodium persulfate > hydrogen peroxide > peracetic acid. Phosphotungstic acid catalyzed hydrogen peroxide and peracetic acid to degrade phenol effectively under near-UV light irradiation or in dark. However, the catalysis of phosphotungstic acid toward sodium persulfate was inhibited for the negative charge repulsion effect. Near-UV light catalyzed phosphotungstic acid to oxidize phenol, but the reaction rate was slow. When phosphotungstic acid and peroxide(hydrogen peroxide or peracetic acid) were both used under near-UV light irradiation, peroxide enhanced the redox cycle of phosphotungstic acid, and phosphotungstic acid promoted the UV light utilizing efficiency of peroxide, and thus phenol degradation rate significantly increased.4. Re-oxidation of a photo-reduced polyoxometalate(POM-) is an important step in many POM-catalyzed reactions. Here we compared the reactivities of several peroxides toward the reoxidation of reduced phosphotungstic acid in the presence of 2-propanol. A unified chain mechanism was proposed and the rate laws were reported for peroxides(XOOX) as oxidants. In the rate-limiting step one-electron transfers from POM- to XOOX to yield a reactive oxygen species ?OX that decomposes the target organic compound. Chain propagation occurs through regeneration of POM to POM- by an organo radical or a transient POM2-. Regeneration of POM- enhances the consumption of organic pollutant relative to the POM- produced. A two-electron reduction of XOOX by POM2- is chain-terminating. The rate constants of one-electron compared to two-electron reduction of XOOX by POM2- depended on XOOX. Two-electron reduction of XOOX is favored for uncharged peroxides(hydrogen peroxide and peracetic acid) but one-electron reduction is favored for charged peroxides(potassium peroxymonosulfate and sodium persulfate). Undoubtedly this is due to charge repulsion between POM2- and the anionic oxidants which was more severe for sodium persulfate.5. A general ionic strength effect was observed for oxidation of reduced phosphotungstic acid(POM-) by sodium persulfate. The net influence of electrolyte may result from a combination of general ionic strength effects, specific cation effects, and specific anion effects. Oxidation of POM- by persulfate was positively affected by added electrolyte in the order, Na Cl >> Na Cl O4 > Na2SO4. A specific Na+ ion enhancement is in effect for the overall reaction, but not for the rate-limiting step. The strong rate-accelerating effect of Na Cl when persulfate was the oxidant was due to short-circuiting the chain by the reaction of chloride ion with sulfate radicals to produce Cl2?- and possibly other reactive halogen species that scavenge POM-.6. Rate-limiting cleavage of the O-O bond in the chain mechanism for POM- oxidation was facilitated by a proton from solution when the leaving group OX- is a strong base(OH-) but not a weak base(O2?-, SO42-, CH3CO2-). Within the tested p H range(1.3 ~ 4.1) rate constant at the rate limiting step followed the order, peracetic acid > peroxymonosulfate > dioxygen > hydrogen peroxide > persulfate at the lower end, and dioxygen > peracetic acid > peroxymonosulfate > hydrogen peroxide > persulfate at the higher end. Dioxygen, while was relatively reactive, produced strongly reactive oxygen species(?OH) in at most 33% yield from POM-. Persulfate, while relatively unreactive, produced many sulfate radicals per molecule of POM- consumed. At p H 4.1, peroxymonosulfate and peroxyacetic acid had similar reactivities, taking XOOX/POM- stoichiometry into account, and were much more reactive than hydrogen peroxide.This study represented the first example demonstrating activation of H2O2 under alkaline condition by phosphate toward oxidation of organic compounds. The mechanism and possible practical applications of peroxides activation by phosphate could be applied in waste water treatment. This dissertation revealed the potential of solid peroxides and solid iron source as Fenton reagent applying in heterogeneous systems(such as soil). We showed the enhancement of polyoxometalate-catalyzed redox reactions toward oxidation of organic compound via using different peroxides as oxidant. The results provided further insight into peroxide-based advanced oxidation processes in organic pollutant decontamination.