Coexistence of Cu, Fe, Pb, and Zn oxides and chlorides as a determinant of chlorinated aromatics generation in municipal solid waste incinerator fly ash.

We investigated chemical determinants of the generation of chlorinated aromatic compounds (aromatic-Cls), such as polychlorinated biphenyls (PCBs) and chlorobenzenes (CBzs), in fly ash from municipal solid waste incineration. The influences of the following on aromatic-Cls formation in model fly ash (MFA) were systematically examined quantitatively and statistically: (i) inorganic chlorides (KCl, NaCl, CaCl2), (ii) base materials (SiO2, Al2O3, CaCO3), (iii) metal oxides (CuO, Fe2O3, PbO, ZnO), (iv) metal chlorides (CuCl2, FeCl3, PbCl2, ZnCl2), and (v) "coexisting multi-models." On the basis of aromatic-Cls concentrations, the ∑CBzs/∑PCBs ratio, and the similarity between distribution patterns, MFAs were categorized into six groups. The results and analysis indicated that the formation of aromatic-Cls depended strongly on the "coexistence condition", namely multimodels composed of not only metal chlorides, but also of metal oxides. The precise replication of metal chloride to oxide ratios, such as the precise ratios of Cu-, Fe-, Pb-, and Zn-chlorides and oxides, may be an essential factor in changing the thermochemical formation patterns of aromatic-Cls. Although CuCl2 acted as a promoter of aromatic-Cls generation, statistical analyses implied that FeCl3 also largely influenced the generation of aromatic-Cls under mixture conditions. Various additional components of fly ash were also comprehensively analyzed.

Thermochemical behavior of lead adjusting formation of chlorinated aromatics in MSW fly ash.

In this study, we examined the thermochemical role of Pb in the formation of chlorinated aromatics (aromatic-Cls) in MSW fly ash at 300-400 °C, a key temperature window for maximum yield. In the presence of lead oxide alone, aromatic-Cls formation was suppressed. One of the mechanisms of suppression was partial chlorination of PbO by an inorganic chlorine source in the solid phase, based on in situ Pb L3-edge X-ray absorption near-edge structure (XANES) data. In contrast, quantitative GC/MS measurements revealed that PbCl2 promoted aromatic-Cls formation to an extent that depends on the Pb concentration, the heating temperature, and the presence of other metal catalysts. We identified two mechanisms of aromatic-Cls formation triggered by PbCl2 in MSW fly ash. First, promotion can occur by the thermochemical partial oxidation of PbCl2. More specifically, real complex solid phase increases the thermochemical oxidation reactivity of PbCl2, based on in situ Pb L3-edge XANES data. Second, Cl K-edge X-ray absorption spectroscopy revealed a coexistent effect of PbCl2 with other metal catalysts such as CuCl2 and FeCl3. The presence of PbCl2 influences the balance of the bonding state of chlorine with Cu and Fe atoms at various temperatures. Thus, Pb in real MSW fly ash functions as an "adjuster" in the generation of aromatic-Cls, the nature of which depends on the lead oxide/chloride ratio and the presence of other metal catalysts.

Role of zinc in MSW fly ash during formation of chlorinated aromatics.

In this study, we determined the thermochemical role of zinc in municipal solid waste (MSW) fly ash. Zinc's role depended on its chemical form and the presence of other metal catalysts. When only zinc was present or it dominated other metal elements, chlorinated aromatic compound (aromatic-Cl) formation was promoted by zinc chloride but blocked by zinc oxide. When only zinc was present, such as in zinc metallurgical plants, some aromatic-Cls were generated and contaminated the environment. When zinc coexisted with other metal promoters in a thermal postcombustion solid phase, such as MSW incineration, Fourier-transform Zn K-edge extended X-ray absorption fine structure (EXAFS) analysis showed that the chemical forms of zinc were primarily chloride and/or oxide, and zinc chloride (ZnCl(2)) was thermally stable in the solid phase. Thus, we used ZnCl(2) in coexistence experiments as a promoter to generate aromatic-Cls. Zinc chloride acted as a coexistent inhibitor of metal catalysis and precursor dimerization to generate aromatic-Cls. There were two coexistent inhibition mechanisms. First, a low-temperature transition of chlorine to the gas phase (low-Cl(g)) occurred with metal catalysts such as CuCl(2) and FeCl(3), confirmed by Cl K-edge near-edge X-ray absorption fine structure (NEXAFS) analysis. Second, X-ray photoelectron spectroscopy (XPS) analysis of the surface or near-surface concentration of ZnCl(2) indicated weak reactivity between the catalysts and the carbon matrix.

Chlorination mechanism of carbon during dioxin formation using Cl-K near-edge X-ray-absorption fine structure.

Many environmental organic chemicals have chloride in their structure. Thus, researching the chlorination mechanism of carbon is of interest. Dioxins are typically concentrated in fly ash collected from the post-combustion zone during the operation of municipal solid waste incinerators. In this study, we report the application of Cl-K near-edge X-ray-absorption fine structure (NEXAFS) in determining the chlorination mechanism of carbon in fly ash. The separation of a chloride-carbon (C-Cl) bond was readily recognizable as a peak in the Cl-K NEXAFS spectrum. Chlorination effects could be estimated using Cl K-edge NEXAFS with no dependence on metal species. Analysis of Cl K-edge NEXAFS spectra showed the reduction of copper(II) chloride at 300°C and oxidation of iron(III) chloride at 400°C in connection with the chlorination of carbon.