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

Formation pathways of polychlorinated dibenzo-p-dioxins and dibenzofurans from burning simulated PVC-coated cable wires

Open burning of PVC-coated cables is a major source of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). In the present study, the formation characteristics of PCDD/F from burning of PVC-based samples with and without metallic copper were evaluated over the dioxin formation temperature window (200-500 °C). This temperature range also inevitably occurs under open burning conditions. The PCDD/F yield from PVC added with Cu increased by factors of 1390 (300 °C), 65 (400 °C) and 17 (500 °C) compared with that from PVC alone, confirming the stimulatory effect of metallic Cu on PCDD/F production. For the first time, a relatively complete isomer-specific analysis is established for PVC acting as source of PCDD/F. Formation pathways of PCDD/F and the reaction mechanisms were investigated using a combined analysis of PCDD/F isomer signatures, thermogravimetric results and Cl K-edge X-ray absorption spectra. De novo synthesis is the major pathway leading to massive production of PCDD/F. Copper extends the temperature range for the concurrence of de(hydro)chlorination of PVC with cross-linking and aromatisation of polyenes and then stimulates cracking of the chlorine-embedded carbon network. Together, these processes contribute to the strongly enhanced formation of PCDD/F via de novo synthesis.

Electrophilic chlorination of dibenzo-p-dioxin and dibenzofuran over composite copper and iron chlorides and oxides in combustion flue gas

Dibenzo-p-dioxin (DD) and dibenzofuran (DF) chlorination mediated by Cu and Fe chlorides can make a direct contribution to the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in combustion flue gas. In this study, a kind of composite Cu and Fe chlorides and oxides (CuFe₉O_9.5Cl₁₀) was prepared by impregnating oxides with HCl solution to imitate the coexistence status of Cu and Fe species in combustion flue gas. Composite CuFe₉O_9.5Cl₁₀ was active in promoting the electrophilic chlorination of DD/DF at 150-300 °C, with the highest activity at 200 °C. DD/DF chlorination could occur under inert atmosphere, and 5% O₂ atmosphere was most favorable for DD/DF chlorination. Electrophilic chlorination of DD/DF primarily favored at 2,3,7,8 positions. Hybridization of Cu and Fe chlorides and oxides not only decreased the starting temperature and activation energy of DD/DF chlorination, but also induced a synergistic effect for accelerating the chlorination of DD/DF. The measured activities of composite CuFe₉O_9.5Cl₁₀ for promoting the chlorination of DD/DF were near to those of composite Cu chloride and oxide (CuO_0.2Cl_1.6), whereas 2 orders of magnitude higher than those of composite Fe chloride and oxide (FeO_0.3Cl_2.4). Comparison of PCDD/F congener distribution patterns indicated that DD/DF chlorination should be a main source of Cl_1-3DFs and Cl_1-2DDs in combustion flue gases.

Thermochemical formation of dioxins promoted by chromium chloride: In situ Cr- and Cl-XAFS analysis

Chromium is commonly found in the flue gases and ashes of Municipal Solid Waste Incineration. It has been reported as an active catalyst for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) during de novo tests, yet its specific mode of action has remained unclear. This study aims to identify the effects of chromium chloride on the formation of PCDD/F and other chloro-aromatics and to elucidate the underlying reaction mechanisms. A series of de novo tests, conducted over a wide range of temperature (from 250 to 550 °C) and for four different oxygen contents (0, 5, 10, 20 %), confirmed the promoting effect of CrCl₃ on the PCDD/F formation. In situ X-ray Absorption Fine Structure (XAFS) spectroscopy was applied to investigate the behavior of CrCl₃ during heating, describing the entire picture of CrCl₃-promoted formation pathways of dioxins. The effect of oxygen was studied by measuring XAFS spectra on samples heated at different oxygen concentrations. According to these spectra, chromium compounds play two key roles during dioxins formation: (a) chlorinating carbon, using chlorine derived from conversion of CrCl₃ into Cr₂O₃, and further oxidation to Cr(VI), and (b) facilitating oxidative destruction of the carbon matrix, while reducing Cr(VI) to Cr₂O₃.

Characterising boiler ash from a circulating fluidised bed municipal solid waste incinerator and distribution of PCDD/F and PCB

In this study, ash samples were collected from five locations situated in the boiler of a circulating fluidised bed municipal solid waste incinerator (high- and low-temperature superheater, evaporator tubes and upper and lower economiser). These samples represent a huge range of flue gas temperatures and were characterised for their particle size distribution, surface characteristics, elemental composition, chemical forms of carbon and chlorine and distribution of polychlorinated dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF) and biphenyls (PCB). Enrichment of chlorine, one of the main elements of organochlorinated pollutants, and copper, zinc and lead, major catalytic metals for dioxin-like compounds, was observed in lower-temperature ash deposits. The speciation of carbon and chlorine on ash surfaces was established, showing a positive correlation between organic chlorine and oxygen-containing carbon functional groups. The load of PCDD/F and PCB (especially dioxin-like PCB) tends to rise rapidly with falling temperature of flue gas, reaching their highest value in economiser ashes. The formation of PCDD/F congeners through the chlorophenol precursor route apparently was enhanced downstream the boiler. Principal component analysis (PCA) was applied to study the links between the ash characteristics and distribution of chloro-aromatics. The primary purpose of this study is improving the understanding of any links between the characteristics of ash from waste heat systems and its potential to form PCDD/F and PCB. The question is raised whether further characterisation of fly ash may assist to establish a diagnosis of poor plant operation, inclusive the generation, destruction and eventual emission of persistent organic pollutants (POPs).

Chemical speciation of lead in secondary fly ash using X-ray absorption spectroscopy

In this study, fly ash samples were collected from bag houses in a Chinese municipal solid waste incinerator (MSWI) and secondary fly ash (SFA) samples were collected from a high-temperature tubular electric furnace by thermal treatment of MSWI fly ash at 1050, 1100, 1150, 1200, and 1250 °C.We determined the speciation and atomic coordinates of lead in SFA using X-ray absorption spectroscopy (XAS) techniques. The results obtained by X-ray absorption near edge structure (XANES) spectra revealed that the mass fraction of PbO in MSWI fly ash was 57.9% (wt %) while PbCl₂ and PbS were the dominant species in SFA. Extended X-ray absorption fine structure (EXAFS) data analysis indicated the atomic coordinates of Pb were proportional to the weights of PbCl₂ and PbS, in good agreement with the XANES spectra. These findings highlight lead evaporation processes in the MSWI fly ash during heat treatment and provide a method for consistent speciation analysis of environmental samples using XAS.

Interplay of metals and bromine with dioxin-related compounds concentrated in e-waste open burning soil from Agbogbloshie in Accra, Ghana

Open burning of electronic waste (e-waste) releases various metals and organohalogen compounds in the environment. Here we investigated the interplay of metals (Cu, Pb, Zn, Fe, Co, and Sr) and bromine (Br) in the formation of dioxin-related compounds (DRCs), including polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (DL-PCBs), as well as non-regulated DRCs such as polybrominated dibenzo-p-dioxins/furans (PBDD/Fs) and their monobrominated PCDD/Fs in soils sampled from open burning e-waste sites at Agbogbloshie in Accra, Ghana. The predominant DRCs were PBDFs, PCDFs, PCDDs, and DL-PCBs. Statistical analyzes, X-ray absorption spectroscopy, and the PCDF/PCDD ratio suggested possible formation paths of PCDD/Fs and DL-PCBs by catalytic behaviors of copper chlorides (CuCl, CuCl2, and Cu2(OH)3Cl) and thermal breakdown of polyvinyl chloride. Predominant formation of brominated furans may be derived from electron transfer from intermediates of PBDE to copper, Cu(II) → Cu(I). Lead chloride also contributed to generate DRCs and may become highly bioaccessible through the open burning of e-waste. The main zinc species (ZnCl2 and ZnS) suggested a possible relationship to generate DRCs and specific zinc source such as tire burning. Cu, Pb, Zn, and Br contained in various e-wastes, wires/cables, plastics, and tires strongly influenced generation of many DRCs.

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.