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Porous carbon-based materials are commonly used to remove various organic and inorganic pollutants from gaseous and liquid effluents and products. In this study, the adsorption of dioxins on both activated carbons and multi-walled carbon nanotube was internally compared, via series of bench scale experiments. A laboratory-scale dioxin generator was applied to generate PCDD/Fs with constant concentration (8.3 ng I-TEQ/Nm). The results confirm that high-chlorinated congeners are more easily adsorbed on both activated carbons and carbon nanotubes than low-chlorinated congeners. Carbon nanotubes also achieved higher adsorption efficiency than activated carbons even though they have smaller BET-surface. Carbon nanotubes reached the total removal efficiency over 86.8 % to be compared with removal efficiencies of only 70.0 and 54.2 % for the two other activated carbons tested. In addition, because of different adsorption mechanisms, the removal efficiencies of carbon nanotubes dropped more slowly with time than was the case for activated carbons. It could be attributed to the abundant mesopores distributed in the surface of carbon nanotubes. They enhanced the pore filled process of dioxin molecules during adsorption. In addition, strong interactions between the two benzene rings of dioxin molecules and the hexagonal arrays of carbon atoms in the surface make carbon nanotubes have bigger adsorption capacity. [ABSTRACT FROM AUTHOR]

Activated carbon (AC) adsorption is an important method to control the emission of dioxins and dioxin-like congeners (polychlorinated dibenzo- p-dioxins and dibenzofurans [PCDD/Fs] and polychlorinated biphenyls [PCBs]), of other semi-volatiles (such as Polycyclic Aromatic Hydrocarbons) and heavy metals (mainly mercury) arising from municipal waste incineration and other effluents. Investigating the adsorption characteristics of AC for PCDD/Fs allows selection of the best AC types, to meet the new and more stringent PCDD/F emission standards. In this study, the adsorption of dioxins on three kinds of commercial AC was evaluated through three bench-scale experiments. A dioxin generator was applied to generate a PCDD/Fs containing gas stream with constant concentration. Physical properties of AC, including its density, Brunauer-Emmet-Teller-surface, pore volume, and pore size distribution, were considered. The results show that the removal efficiencies of PCDD/Fs correlate more strongly with pore volume ( r2>0.93) than with the surface area ( r2<0.48). Among the three types of AC tested, the coconut shell sample with wide pore size distribution reached the highest adsorption efficiency. Under the testing conditions used, the total removal efficiency reached over 96%, much more than the 81.7% efficiency achieved by lignite AC. In addition, the selectivity of lignite AC for 17 toxic PCDD/F congeners is distinct from the two other samples. [ABSTRACT FROM AUTHOR]

Abstract: High temperature combustion experiments of waste printed circuit boards (PCBs) were conducted using a lab-scale system featuring a continuously-fed drop tube furnace. Combustion efficiency and the occurrence of inorganic bromine (HBr and Br2) were systematically studied by monitoring the main combustion products continuously. The influence of furnace temperature (T) was studied from 800 to 1400°C, the excess air factor (EAF) was varied from 1.2 to 1.9 and the residence time in the high temperature zone (RTHT) was set at 0.25, 0.5, or 0.75s. Combustion efficiency depends on temperature, EAF and RTHT; temperature has the most significant effect. Conversion of organic bromine from flame retardants into HBr and Br2 depends on temperature and EAF. Temperature has crucial influence over the ratio of HBr to Br2, whereas oxygen partial pressure plays a minor role. The two forms of inorganic bromine seem substantially to reach thermodynamic equilibrium within 0.25s. High temperature is required to improve the combustion performance: at 1200°C or higher, an EAF of 1.3 or more, and a RTHT exceeding 0.75s, combustion is quite complete, the CO concentration in flue gas and remained carbon in ash are sufficiently low, and organobrominated compounds are successfully decomposed (more than 99.9%). According to these results, incineration of waste PCBs without preliminary separation and without additives would perform very well under certain conditions; the potential precursors for brominated dioxins formation could be destroyed efficiently. Increasing temperature could decrease the volume percentage ratio of Br2/HBr in flue gas greatly. [Copyright &y& Elsevier]

Commercial catalysts for the catalytic oxidation of chloroaromatics are mainly based on either noble metals or transition metal oxides supported on a suitable carrier. This paper reviews studies relative to these two generic groups of catalysts and their performances for chloroaromatic decomposition, i.e. conversion efficiency, stability and selectivity towards harmless products (e.g., CO 2 ), and in particular, avoiding the formation of polychlorinated by-products. The various approaches towards improved catalysts, including tuning the support, adding dopants or promoters, improving the preparation methods, or auxiliary means such as the introduction of ozone (O 3 ) or hydrogen peroxide (H 2 O 2 ) in the gas to be treated, and combination with non-thermal plasma treatment are systematically reviewed. This review also appraises various modes of deactivation (i.e., originating from fouling, coking, poisoning, sintering of the catalyst, and from volatilization of its active phases) as well as possible methods for regeneration. [ABSTRACT FROM AUTHOR]

Abstract: Supercritical water oxidation (SCWO), with hydrogen peroxide as oxidant, is applied to the co-disposal of two distinct waste streams: municipal solid waste leachate and incineration fly ash. The chemical oxygen demand (COD) removal efficiency increases rapidly with rising temperature and excess oxygen. Rising residence time from 1 to 2min has surprisingly little effect. The addition of fly ash accelerates COD conversion markedly and also polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs, dioxins) in the original fly ash are efficiently destroyed. High-chlorinated PCDD/Fs are more likely to be destroyed than low-chlorinated PCDD/Fs, at all experimental conditions. In addition, PCDDs are much more reactive than PCDFs, since the PCDDs/PCDFs ratio declines from 0.17 to 0.12 as excess oxygen rises from 0% to 300%. [Copyright &y& Elsevier]