Destruction kinetic of PCDDs/Fs in MSWI fly ash using microwave peroxide oxidation.

Microwave peroxide oxidation is a less greenhouse gas emission and energy-efficient technology to destroy toxic organic compounds in hazardous waste. The research novelty is to adopt the innovative microwave peroxide oxidation in H2SO4/HNO3 solution to efficiently destroy the polychlorinated dibenzo-p-dioxins (PCDDs)/Fs in municipal solid waste incineration fly ash. The major objective of this paper is to study dynamic destruction of PCDDs/Fs using the microwave peroxide oxidation. Almost all PCDDs/Fs in the raw fly ash can be destructed in 120 min at a temperature of 423 K using the microwave peroxide oxidation treatment. It was found that the microwave peroxide oxidation provides the potential to destruct the PCDDs/Fs content in municipal solid waste incinerator (MSWI) fly ash to a low level as a function of treatment time. A useful kinetic correlation between destruction efficiency and treatment conditions is proposed on the basis of the experimental data obtained in this study. The significance of this work in terms of practical engineering applications is that the necessary minimum treatment time can be solved using a proposed graphic illustration method, by which the minimum treatment time is obtained if the desired destruction efficiency and treatment temperature are known. Because of inorganic salt dissolution, the temperature would be a critical factor facilitating the parts of fly ash dissolution. Material loss problem caused by the microwave peroxide oxidation and the effects of treatment time and temperature are also discussed in this paper.

Degradation mechanism of PCDDs initiated by OH radical in Photo-Fenton oxidation technology: quantum chemistry and quantitative structure-activity relationship.

A detailed understanding of the degradation mechanism of polychlorinated dibenzo-p-dioxins (PCDDs) is of great necessity. In wastewater treatment, an important degradation process of PCDDs ascribes to its reaction with the photo-Fenton reagent. In this paper, the reaction of 2,3,7,8-TeCDD with OH radicals has been studied using high level molecular orbital theory. The profile of the potential energy surface is constructed. A complete description of the possible degradation mechanism in solution is provided. Two degradation mechanisms are proposed: ring-opening and adducting mechanism, adducting and ring-opening mechanism. The main products obtained are 4,5-dichlorinate-o-dihydroxybenzene and 4,5-dichlorinate-o-quinone. The study on the quantitative structure-activity relationship of these PCDDs is performed. The structure-activity model has been constructed, in which three structural parameters, RO5-C12, RO5-C13 and QC14, are found to be positively correlated to the degradation activities.

Photodegradation of polychlorinated dibenzo-p-dioxins: comparison of photocatalysts.

1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (1,2,3,6,7,8-HxCDD) and octachlorodibenzo-p-dioxin (OCDD) were photocatalytically degraded using immobilized TiO(2), ZnO and SnO(2) films under ultraviolet (UV) with the wavelength between 300 and 450 nm. The specific surface areas of TiO(2), ZnO and SnO(2) powders were calculated as 54.2, 4.6 and 4.8 m(2)/g, and the band gap energies were determined to be 3.17, 2.92 and 4.13 eV, respectively. The light source used had wavelengths between 300 and 450 nm, and the main wavelength was approximately 365 nm; hence, the smaller quantum efficiency of SnO(2) with wider band gap due to poor utilization of the UV light was expected. X-ray diffraction (XRD) revealed that sintered photocatalysts at 400 degrees C did not alter their characteristics. The first-order rate constants of OCDD in UV/TiO(2), UV/ZnO and UV/SnO(2) were 5.30, 0.74 and 0.28 h(-1), respectively; those of 1,2,3,6,7,8-HxCDD in UV/ZnO and UV/SnO(2) were 3.28 and 3.19 h(-1), respectively. As expected, photodegradation rates declined as the number of chlorine atoms increased. Due to the low dosage (50 ng) of target compounds used, the amounts of Cl(-) and total organic carbon were too small for quantification. No 2,3,7,8-substituted congeners was identified during the photodegradation, and the UV/photocatalyst treatment might offer an effective treatment for PCDDs.

Photodegradation of PCDD/Fs adsorbed on spruce (Picea abies (L.) Karst.) needles under sunlight irradiation.

Spruce (Picea abies (L.) Karst.) needles were exposed to exhaust gas containing polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) generated by combustion of polyvinyl chloride, wood, high-density polyethylene, and styrene. Photodegradation of PCDD/Fs adsorbed on spruce needles under sunlight irradiation was studied. The photodegradation of PCDD/Fs follows pseudo-first-order reaction kinetics, with photolysis half-lives ranging between 40 and 100 h. The photolysis rates of PCDF congeners are higher than PCDD congeners with the same chlorinated substitutions. Higher chlorinated PCDD/Fs tend to photolyze slowly. The wax components in spruce needles may act as proton donors and accelerate the photolysis rate. C-Cl cleavage through the addition of protons to PCDD/F molecules may be an important route for PCDD/Fs photodegradation.

Quantitative structure-property relationships (QSPRs) on direct photolysis quantum yields of PCDDs.

By the use of partial least squares (PLS) method and 16 fundamental quantum chemical descriptors computed by PM3 Hamiltonian, quantitative structure-property relationships (QSPRs) were obtained for direct photolysis quantum yields of selected polychlorinated dibenzo-p-dioxins (PCDDs). Direct photolysis quantum yields for PCDDs without experimental quantum yield values were predicted. The QSPR results showed that it was mainly the number of chlorine atoms bonded to the parent structure, the largest positive atomic charge on a chlorine atom, the dipole moment, and the frontier molecular orbital energies (Ehomo and Elumo) that determine the direct photolysis quantum yields of the PCDDs. Increasing the number of chlorine atoms, dipole moment, and the largest positive atomic charge on a chlorine atom, leads to decrease of photolysis quantum yields. Increasing Elumo, Ehomo and Elumo - Ehomo values lead to increase of log Y values.

Photodecomposition of 1,2,3,4- and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in water-alcohol media on a solid support.

We used a hydrophobic solid support, octadecylsilylated silica gel (C18), packed in a quartz column as a reaction medium for the photolysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD). When we exposed the column to a 450 W UV lamp, the adsorbed 1,2,3,4-TCDD or 2,3,7,8-TCDD in 10% 2-propanol/water decomposed completely in 20 minutes and 5 minutes, respectively. The large estimated partition coefficient of 1,2,3,4-TCDD in 10% 2-propanol/water (> 1000) indicates that on the C18 stationary phase, both the saturated hydrocarbon chains and the absorbed 2-propanol may act as proton donors and accelerate the photolysis. In direct sunlight, the adsorbed 1,2,3,4-TCDD in 10% 2-propanol/water decomposed much faster than in a nonaqueous solvent (50% 2-propanol/methanol). This solvent effect is advantageous for the practical use of the C18 photolysis process in aqueous waste treatment. We have demonstrated that complete C18 trapping with continuous photodecomposition of TCDD contained in an aqueous alcohol waste is possible.

Decomposition of toxic and environmentally hazardous 2,3,7,8-tetra-chlorodibenzo-p-dioxin by gamma irradiation.

The decomposition of the toxic and environmentally hazardous 2378-TCDD by gamma irradiation was studied and successfully used to decontaminate laboratory wastes containing small quantities of this chemically and biologically stable compound. The method makes use of gamma irradiation from a commercial 60cobalt facility at high dose levels (1000 kGy) to break down the compound into nontoxic products. Irradiation also decomposed 2378-TCDD in contaminated soil from the Seveso accident.