Formation Pathways of Mono- to Octa-Chlorinated Dibenzo-p-dioxins and Dibenzofurans in Main Organochemical Industries

Formation and Inventory of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans and Other Byproducts along Manufacturing Processes of Chlorobenzene and Chloroethylene

Chlorinated organic chemicals are produced and used extensively worldwide, and their risks to the biology and environment are of increasing concern. However, chlorinated byproducts [e.g., polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs)] formed during the commercial manufacturing processes and present in organochlorine products are rarely reported. The knowledge on the occurrences and fate of unintentional persistent organic chemicals in the manufacturing of organochlorine chemical is necessary for accurate assessment of the risks of commercial chemicals and their production. Here, PCDD/Fs were tracked throughout chlorobenzene and chloroethylene production processes (from raw materials to final products) by target analysis. Other byproducts that can further transform into PCDD/Fs were also identified by performing non-target screening. As a result, the PCDD/F concentrations were mostly the highest in bottom residues, and the octachlorinated congeners were dominant. Alkali/water washing stages may cause the formation of oxygen-containing byproducts including PCDD/Fs and acyl-containing compounds, so more attention should be paid to these stages. PCDD/Fs were of 0.17 and 0.21-1.2 ng/mL in monochlorobenzene and chloroethylene products, respectively. Annual PCDD/F emissions (17 g toxic equivalent in 2018) during chlorobenzene and chloroethylene production were estimated using PCDD/F emission factors. The results can contribute to the improvement of PCDD/F inventories for the analyzed commercial chemicals.

Exposure levels of PCDD/Fs and PCBs in human blood and the transplacental transfer characteristics in cord blood of newborns near the industrialized area

Mono-to octa-chlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs) in pooled blood from the general population living near a typical industrialized area were investigated. Less chlorinated PCDD/Fs (mean: 2602 pg L^-1) were 7.5 times those of highly chlorinated ones (mean: 349 pg L^-1). The average ΣPCBs and Σdl-PCBs concentrations in human (cord) blood were 2741 (117) and 18 (0.31) ng L^-1, respectively. Higher concentrations of highly chlorinated PCDD/Fs were found in females than in males across different ages. The mean concentrations (and toxic equivalents (TEQs)) of PCDD/Fs were 282 (27) pg L^-1 in males and 312 (32) pg L^-1 in females. The concentrations of the PCDD/Fs and PCBs increased with age for both males and females, which might be caused by the long half-lives of these compounds and decreases in metabolic rates with age as the metabolic of nutrients, food, and also PCDD/Fs and PCBs would trend to slow. The TEQ of total PCDD/Fs and PCBs was higher in blood from orthopedics patients (107 pg L^-1) than other patients. This result may be associated with the bone density and pollutant bioaccumulation. In addition, total concentration of PCDD/Fs and PCBs in blood of women at reproductive age were 6.6 and 37 times the cord blood of newborns, respectively. Positive correlation of PCDD/Fs and PCBs especially for the higher chlorinated compounds between female and cord blood were discovered, which might be caused by the transplacental transfer characteristics and blood barrier for macromolecules and reduce the chemical exposure risks for newborns.

Microbial reductive dechlorination of polychlorinated dibenzo-p-dioxins: Pathways and features unravelled via electron density

Microbial reductive dechlorination provides a promising approach for remediating sites contaminated with polychlorinated dibenzo-p-dioxins (PCDDs). Nonetheless, the overall dechlorination pathways and features remain elusive. Herein, we address these issues by quantum chemical calculations, considering the calibrations of reductive dechlorination of 15 PCDDs mediated by three Dehalococcoides strains. Chlorine substituents with lower electron density are prone to be microbially abstracted, which differentiates 72 microbe-active PCDDs from 3 nonactive analogues with a success rate of 100%. For all 256 transformation routes of 75 PCDDs, electron density differences of chlorines pinpoint 105 viable and 125 unviable pathways, corresponding a success rate of 90%. The feasibility of 26 reductive dechlorination pathways are uncertain because of the limited available experimental data. 98% (251/256) of microbial chlorine abstraction follows an order of Cl_O,Cl>Cl_Cl,Cl>Cl_H,O>Cl_H,Cl>Cl_H,H=0. PCDDs solely containing chlorines at C₁, C₄, C₆, and/or C₉ can be completely dechlorinated to non-chlorinated dioxin; while PCDDs housing chlorines at C₂, C₃, C₇, and/or C₈ can be dechlorinated to 2-MCDD or 2,7/8-DCDD as final products. These findings also support reductive dechlorination of PCDDs in mixed cultures and sediments (> 98% and 83%). These findings would promote the application of dechlorinating bacteria in targeted remediation and facilitate the respective studies on other POPs.

Global status of dioxin emission and China's role in reducing the emission

The global status of dioxin emissions across 150 countries/regions were compiled in this study. China, the major emitter of dioxin and the largest developing country, was chosen as an example to illustrate its emission reductions. The global dioxin emissions were about 97.0 kg TEQ/year, Asia and Africa emitted the most dioxins among the continents. Globally, open burning processes were the most important sources of dioxins. Dioxin emissions in developed countries have remained at low and stable level, while those in developing countries have remained at relatively high level or have continued to increase in recent years. It can be speculated that the global dioxin emissions will increase first and then decrease in the future. Chinese dioxin emissions were stable around 9 kg toxic equivalent (TEQ) in recent years, while 17 subcategories are the key sources of dioxin control in the future. Moreover, according to analysis toward China's dioxin emission trend and sources, there is a large space for dioxins reduction in industries such as metal production, waste incineration and disposal. The results indicated that there is at least 30-70% of reduction scope in China based on three scenarios, and this will reduce the world's annual dioxin emissions by 2.7-6.8%.

Measurement of Dioxin Emissions from a Small-Scale Waste Incinerator in the Absence of Air Pollution Controls

Polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/Fs) emissions from basic small-scale waste incinerators (SWI) may cause health risks in nearby people and are thus subject to stringent regulations. The aim of this study was to evaluate PCDD/F emission and reduction of a basic SWI in the absence of air pollution controls (APCs). The results indicated that the stack gas and fly ash presented average PCDD/F levels and emission factors of 3.6 ng international toxic equivalent (I-TEQ)/Nm³ and 189.31µg I-TEQ/t and 6.89 ng I-TEQ/g and 137.85µg I-TEQ/t, respectively, much higher than those from large municipal solid waste incinerators (MSWI). PCDD/Fs congener fingerprints indicated that de novo synthesis played a dominant role in the low-temperature post-combustion zone and increased the presence of high-chlorine substituted congeners. On the basis of the emission factor 327.24 µg I-TEQ/t-waste, approximately 3000 g I-TEQ dioxins might be generated in total through basic SWIs and open burning. After refitting an SWI by adding activated carbon injection with a bag filter (ACI+BG), the PCDD/F emissions decreased to mean values of 0.042 ng I-TEQ/Nm³, far below the standard of 0.1 ng I-TEQ/Nm³, and the removal efficiency reached 99.13% in terms of I-TEQ. Therefore, it is entirely feasible to considerably reduce PCDD/F emissions by refitting basic SWI, which is positive for the future development of rural solid waste (RSW (RSW) disposal by SWI.

Emission profiles and formation pathways of 2,3,7,8-substituted and non-2,3,7,8-substituted polychlorinated dibenzo-p-dioxins and dibenzofurans in secondary copper smelters

Secondary copper smelting production is one of the largest polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) emission sources in the world. However, the formations and emissions of non-2,3,7,8-PCDD/Fs have rarely been studied. Toxicology and metabolism studies have proved that non-2,3,7,8-PCDD/Fs may also be toxic to mammals. To better explore the pathways and mechanisms involved in transformation among non-2,3,7,8-PCDD/F and 2,3,7,8-PCDD/F congeners, their full picture was investigated in stack gas and fly ash samples collected in typical secondary copper smelting plants. The concentration ranges for 2,3,7,8-PCDD/Fs and non-2,3,7,8-PCDD/Fs in the stack gas samples were 0.09-5.24 ng/Nm³ and 0.11-7.47 ng/Nm³, respectively. The corresponding concentration ranges in the fly ash samples were 20-2712 ng/g and 2.7-818 ng/g. PCDD/F emissions were mainly from the oxidation stage, and these emissions contributed to 42.6-44.8% of the total emissions from the secondary copper smelting processes. Lower chlorinated PCDD/Fs partitioned more into the stack gas, whereas higher chlorinated PCDD/Fs were more likely to concentrate in the fly ash. Non-2,3,7,8-PCDD/Fs were more likely than 2,3,7,8-PCDD/Fs to associate with the gas phase. Chlorination transformation may occur among PCDD congeners, including 2,3,7,8-PCDD and non-2,3,7,8-PCDD congeners.

Monochlorinated to Octachlorinated Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Emissions in Sintering Fly Ash from Multiple-Field Electrostatic Precipitators

Polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) emissions in fly ash from multiple-field electrostatic precipitators in different sized sintering plants were studied. The monochlorinated-trichlorinated and tetrachlorinated-octachlorinated PCDD/F concentrations were higher for small plants (90 m²) than for medium (91-180 m²) and large (>180 m²) plants. The PCDD/F concentrations and less-chlorinated PCDD/F contributions to the total PCDD/F concentrations increased as the fly ash particle size decreased moving through the precipitator stages; the abundance of monochlorinated-trichlorinated PCDD/F congeners and homologues also increased. The ash particle size and surface area can be directly used to indicate monochlorinated-trichlorinated PCDD/Fs and toxic equivalents (TEQs). Previously ignored PCDD/F emissions in discarded fly ash were identified. Estimated total monochlorinated-trichlorinated PCDD/F and TEQ emissions in discarded fly ash were 155 and 1.979 kg TEQ, respectively, in 2003-2014, and the ratio between annual PCDD/F emissions in discarded fly ash and flue gases has gradually increased. Reductions in monochlorinated-trichlorinated PCDD/F emitted in flue gas and fly ash in 2003-2014 were 28 and 40 kg, respectively, because of the phasing out of small-scale plants. Reductions in TEQs emitted in flue gas and fly ash in 2003-2014 were 7476 and 180 g TEQ, respectively.

Theoretical and experimental formation of low chlorinated dibenzo-p-dioxins and dibenzofurans in the Fenton oxidation of chlorophenol solutions

The formation of chlorinated and non-chlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) has been experimentally investigated after the Fenton oxidation of 2-chlorophenol (2-CP, 15.56 mM) aqueous solutions by assessing the influence of iron concentration (0.09-2.88 mM), hydrogen peroxide dose (40.44-202.20 mM), temperature (20-70 °C) and chloride concentration (0-56.35 mM). The presence of chloride in the medium together with room temperature and substoichiometric Fenton conditions (40.44 mM H2O2) led to an increase in total PCDD/Fs concentration from less than 1 ng L(-1) to 2 μg L(-1). Results showed a dominance of the dichlorinated species (DCDD/Fs) in the homologue profile of total PCDD/Fs reaching values up to 1.5 μg L(-1). Furthermore, the products distribution exhibited a gradual decrease in the homologue concentration as the chlorination degree increased from di-to octachloro-substituted positions. Considering the characteristics of the reaction medium, the experimental results, and the information gathered in bibliography with regard to the generation of active radicals from 2-chlorophenol, a mechanism describing the formation of low chlorinated PCDD/Fs in a Fenton oxidizing aqueous system has been proposed.

Influence of long-range atmospheric transportation (LRAT) on mono-to octa-chlorinated PCDD/Fs levels and distributions in soil around Qinghai Lake, China

Long-range atmospheric transportation (LRAT) of persistent organic pollutants followed by their deposition in cold, arid regions is of wide concern. This problem occurs at Qinghai Lake in the northeastern Tibetan Plateau, a sparsely populated area with extreme weather conditions and little current or historical anthropogenic pollution. The concentrations and distribution patterns of the mono-to octa-chlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) congeners in surface soil samples collected from around Qinghai Lake were quantified. Concentration differences between low-(mono-to tri-) chlorinated PCDD/Fs and high-(tetra-to octa-) chlorinated PCDD/Fs were measured. High PCDD/F levels were detected, with total concentrations of 15,108 ± 6323 pg/g for the 27 PCDD/F congeners and 15,104 ± 6324 pg/g for the low-chlorinated PCDD/Fs. The concentrations of 17 2,3,7,8-substituted PCDD/Fs were only 3.1 ± 4.4 pg/g and the corresponding international toxicity equivalency (I-TEQ) was 0.11 ± 0.22 pg I-TEQ/g. Given their higher vapor pressures and lower boiling points, low-chlorinated PCDD/Fs, were predominantly gaseous, whereas high-chlorinated PCDD/Fs were predominantly solid, indicating that there is a higher potential for long-range transport of low-chlorinated PCDD/Fs. Overall, because of their high LRAT potential, low-chlorinated PCDD/Fs may pose a greater risk to local ecosystems in cold, remote areas than high-chlorinated PCDD/Fs.

Mono- to Octachlorinated Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Emissions from Sintering Plants Synergistically Controlled by the Desulfurization Process

The influence of desulfurization systems in sintering plants on polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) concentrations, profiles, and emission factors was studied. Mono- to tri-CDD/Fs and tetra- to octa-CDD/F concentrations were 4.4 ± 2.3 and 10.5 ± 8.3 ng m(-3), respectively, at the inlets and 0.87 ± 0.48 and 0.47 ± 0.22 ng m(-3), respectively, after desulfurization. The toxic equivalents (TEQs) were 0.95 ± 0.093 and 0.51 ± 0.040 ng of I-TEQ m(-3) at the inlets and after desulfurization, respectively. The congener profiles and homologue distributions were dominated by 2-MoCDF and MoCDF, respectively. The PCDD/F removal efficiencies achieved by desulfurization increased as the chlorination level increased. The PCDD/Fs became adsorbed to gypsum. Annual mono- to tri-CDD/Fs PCDD/F and TEQ (tetra- to octa-CDD/F) emission factors for flue gas and gypsum between 2003 and 2012 were determined. The total amounts of mono- to tri-CDD/Fs emitted in flue gas and gypsum between 2003 and 2012 were 10.7 and 10.2 kg, respectively. The total TEQs emitted in flue gas and gypsum between 2003 and 2012 were estimated to be 15486 and 1878 g of I-TEQ, respectively. PCDD/Fs adsorbed to gypsum are not effectively eliminated. The PCDD/F concentrations increased as the fly ash surface area increased moving through the electrostatic precipitator stages.