Article pubs.acs.org/est
Characterization and Inventory of PCDD/F Emissions from the Ceramic Industry in China Mang Lu,*,† Guoxiang Wang,‡ Zhongzhi Zhang,§ and Youming Su∥ †
School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, Jiangxi Province, China College of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, Hunan Province, China § State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China ∥ Offshore Oil Engineering Co., Ltd., Engineering Company, Tianjin 300451, China ‡
S Supporting Information *
ABSTRACT: The ceramic industry is considered to be a potential source of dioxins (polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), considering the widespread distribution of dioxins in kaolinitic clays. Nevertheless, studies on the emission of dioxins from the ceramic industry are still very scarce. In this study, raw clays and stack gases from six typical ceramic plants in China were collected and analyzed to estimate the emission of dioxins from the ceramic industry. Dioxin profiles in raw clays were characterized by the domination of the congener octachlorodibenzo-p-dioxin (OCDD), and the contents of other congeners declined with the decreasing degree of chlorination. During the ceramic firing process, a considerable amount (16.5−25.1 wt % of the initial quantity in raw clays) of the dioxins was not destroyed and was released to the atmosphere. Dechlorination of OCDD generated a broad distribution within the PCDD congeners including a variety of non-2,3,7,8-substituted ones with the mass abundance of 0.4−3.6%. Based on the mean concentrations measured in this study, the inventory of PCDD/Fs from the manufacturing of ceramics on the Chinese scale was estimated to be 7.94 kg/year; the corresponding value on the I-TEQ basis is 133.6 g I-TEQ/year. This accounts for about 1.34% (I-TEQ basis) of the total emission of dioxins to the environment in China. The results suggest that the ceramic industry is a significant source of dioxins in the environment.
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INTRODUCTION Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) (hereafter referred to as dioxins) are among the most toxic organic compounds in the environment due to their potency as aryl hydrocarbon receptor ligands. These persistent organic pollutants (POPs) have both natural and anthropogenic origins.1−3 Since being detected for the first time in Tertiary ball clays of the Mississippi embayment in 1996,4 the presence of PCDD/Fs from natural sources has been confirmed in numerous kaolins and sedimentary kaolinitic clays in North and South America, Europe, South Pacific, and Asia.5 Rappe and Anderson6 found the U.S. ball clays contained averagely 1035 pg/g WHO-TEQ PCDD/Fs, whereas the German kaolin exhibited a much lower content of 198 pg/g WHO-TEQ. In contrast, kaolins from Georgia and North Carolina contained PCDD/Fs close to the limit of detection (LOD). Abad et al.7 analyzed two kaolin samples among other types of clay, and found that the PCDD/F contents of the kaolins were almost the same as those of German ball clays with values of 232 and 461 pg/g WHO-TEQ. © 2012 American Chemical Society
The congener compositions were fairly consistent among the clays from the various geographical regions despite strong variations in the content of PCDD/Fs.5,8 As a class of POPs exhibiting the most toxicity and posing the most serious health hazard, PCDD/Fs have been extensively studied and various emission sources have been identified and evaluated. It has been demonstrated that a variety of anthropogenic actions contribute to the emissions of dioxins such as the steel industry,9−11 metallurgical industry,12,13 coking industry,14,15 and waste incineration.16−18 The emissions of dioxins from municipal waste incineration are decreasing, due to the implementation of strict regulations and technological progress.15 Thus, the identification and quantification of other potential emission sources of dioxins is worth studying, which is one of the key steps for controlling and regulating their emissions and for effectively decreasing further environmental exposure to dioxins around the world.19 Received: Revised: Accepted: Published: 4159
December 23, 2011 February 27, 2012 March 5, 2012 March 5, 2012 dx.doi.org/10.1021/es204639x | Environ. Sci. Technol. 2012, 46, 4159−4165
Environmental Science & Technology
Article
Table 1. Basic Information of the Six Ceramic Plants, Characteristics of the End Products, and Details of Gas Sampling
annual clay mixture consumption ( × 104 tons) average flow rate ( × 104 Nm3/h) operating time per year (h) air pollution control device sampling date sampling rate (L/min) sample volume collected (Nm3) a
CP-1
CP-2
CP-3
CP-4
CP-5
CP-6
floor tile
floor tile
wall tile
floor tile
floor tile
wall tile
25.6 33.8a 22.6b 8400 fabric filter 06/03/2011 24.5 8.82
31.5 40.2a 25.3b 8400 fabric filter 8/03/2011 26.3 9.47
11.3 15.6a 10.6b 8400 wet scrubber 14/03/2011 23.4 8.42
26.3 34.1a 21.5b 8400 fabric filter 19/03/2011 27.5 9.90
22.8 30.4a 18.5b 8400 wet scrubber 22/03/2011 25.2 9.07
15.7 20.5a 13.3b 8400 wet scrubber 24/03/2011 26.8 9.65
Preheating and firing zone stack. bCooling zone stack.
of a heated probe liner, filter box equipped with a glass fiber filter, and a water-cooled XAD-2 adsorbent trap. Before sampling, the XAD-2 was spiked with a known quantity of U.S. EPA Method 23 surrogate standard solution, containing five surrogate 13C12− PCDD/Fs (37Cl4−PCDD/Fs) target compounds (Wellington Laboratories, Ontario, Canada). The samples were tightly wrapped individually with aluminum foil (to prevent pollution and loss) and sealed in labeled resealable bags. Once the sampling was completed, the samples were immediately brought back to the laboratory under refrigeration. Sample Preparation. The clay mixture and fired tiles were ground into fine particles using a superfine pulverizer (CAX, Shanghai Yuanhua Machinery Co., Ltd., China). Analysis of PCDD/Fs was carried out based on U.S. EPA Method 23 and 1668A modifications. In brief, the particles and XAD-2 were spiked with a known amount of 13C12−PCDD/Fs internal standards (Wellington Laboratories, Ontario, Canada). The samples were extracted using toluene for 24 h in a Soxhlet apparatus, and the extracts were concentrated on a rotary evaporator. The extracts were then serially purified using acidic silica-gel columns [44 wt % sulfuric acid-impregnated silica gel] and 10 wt % silver nitrate-impregnated silica gel.5,15 The analyte was fractionated using active carbon column (50 wt % active carbon-blended silica gel and 50 wt % active carbon dispersed silica gel). The column was eluted with toluene after a passage of hexane. The toluene fraction that contained PCDD/Fs was concentrated to almost dryness by rotary evaporator under nitrogen, and 13C12-labeled PCDD/F standards were added into the corresponding fractions to calculate the recoveries. Chemical and Mineralogical Analysis. Element compositions such as Si, Al, Fe, K, Ti, and Mg were determined in clays using energy dispersive X-ray spectroscopy (Horiba Co., Kyoto, Japan) equipped with scanning electron microscope (Hitachi High-Technologies Co., Tokyo, Japan).21 The qualitative mineral content of the clay sample was determined via X-ray diffraction using random powdered samples and 10 000 under SIM mode. Ion source temperature was set as 300 °C. The temperature of GC-MS interface was set
China is the largest ceramic producing country in the world, accounting for approximately 90% of global ceramic production in 2010, with many ceramic plants of different scales and techniques being in operation. Although the existence of dioxins in clays has been well documented, few studies on their emissions during ceramic manufacturing have been reported to the best of our knowledge. Ferrario and Byrne20 reported very trace levels of PCDD/Fs were present in the final fired ceramic products. Because large discrepancies in scales and techniques exist for the global ceramic industry, the development of dioxins inventory from ceramic plants in China is significant for evaluating release of dioxins from the ceramic industry in the world. The main objective of this study was to assess the amount of PCDD/F release from the ceramic industry in China. For this purpose, six typical ceramic plants were investigated in China. Emission levels and characteristics are presented and discussed. Emission factors of these POPs were also derived and used to estimate the emissions of these dioxins from the ceramic industry. These data may be helpful for understanding the extent of dioxin release threat and the contribution of dioxins from the ceramic industry and developing an emission inventory of PCDD/Fs.
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MATERIALS AND METHODS Sampling. A number of different energy sources such as electricity, liquefied petroleum gas, coal gas, fuel oil, and natural gas are used as energy sources in ceramic kilns. Six ceramic plants (numbered CP-1 to CP-6) fuelled by natural gas were selected as the research sites. Foregoing analysis showed that dioxins were not detected from the natural gas used, eliminating potential dioxin emission from fuel combustion. Hence, the raw clay was identified as the main source of dioxins. Table 1 presents the basic information for the six plants and characteristics of the end products (tiles). Figure S1 (Supporting Information) schematically illustrates one of these kilns. Emission of dioxins may occur during preheating and cooling of tiles in the firing process. The tiles were subjected to the standard thermal cycles used at the plants where the tests were carried out, with total firing times (cold to cold) of 55−70 min, and peak firing temperatures of about 1200−1300 °C. To estimate the emission contribution, the gas samples after the gas purification system were simultaneously collected from preheating and firing zone stack and cooling zone stack. The details of gas sampling are list in Table 1. The stack gas samples were withdrawn using an automatic isokinetic sampling system (Clean Air Instrumentation, France) according to U.S. EPA Method 23. The sampling train consisted 4160
dx.doi.org/10.1021/es204639x | Environ. Sci. Technol. 2012, 46, 4159−4165
Environmental Science & Technology
Article
to 300 °C. Ionization current was 500 μA; electron impact source voltage was 38 eV; ion acceleration voltage was 10 kV; and mass correction mode was lock mass mode. Quality Assurance. Field and laboratory blanks were routinely monitored and in most cases were under the LOD. The mean recoveries of surrogate standards ranged from 79% to 104% with a relative standard deviation (RSD) less than 12%, which is within the acceptable 70% to 130% range set by the U.S. EPA Method 23. The samples were extracted with a blank, which consisted of a Soxhlet thimble containing anhydrous sodium sulfate. Method detection limit and quantification limit values were calculated from the variance associated with replicate analysis (n = 5), and the details are available in the Supporting Information (Table S1). The international toxic equivalent (I-TEQ) was calculated using the international toxic equivalence factor.22 In the case of values below the LOD, I-TEQ was calculated by using the half of the LOD. The concentrations are presented in picogram per cubic meter (pg/Nm3, standard conditions) for gaseous samples and picogram per gram (pg/g, oven-dried basis) for solid samples.
of impurities before and after elutriation.5 Generally, after being transported to the plant, the raw material is mined, sieved, dried, and homogenized for ceramic production. In this study, the concentrations of dioxins in raw and processed material were determined and results showed negligible differences. Hence, the determined dioxin contents in product materials were purely derived from raw clays, instead of from production/ preparation processes. The concentrations of individual toxic (2,3,7,8-substituted) PCDD/F congeners in the raw and processed clay mixture from each plant, along with their I-TEQ values are summarized in Table 3 and Table S2 (Supporting Information), respectively. As shown, the higher chlorinated PCDDs were dominant, whereas the PCDF contents were close to or below the LOD. To evaluate the distribution of PCDD/F patterns in clays, the concentrations of PCDD/F congeners were normalized to the percentage of the sum of PCDD/Fs and plotted in Figure 1. It can be observed that the distribution and the congener profiles were remarkably similar among all samples analyzed. The PCDD/F congener pattern was very similar to that obtained by numerous authors.3−5,8 As shown in Table 3 and Figure 1, octachlorodibenzo-p-dioxin (OCDD) was the most abundant congener of PCDD/Fs in all samples, which took up 60−70% of the total PCDD/Fs. Generally, concentrations decreased in the following order: OCDD > heptachlorodibenzo-p-dioxins (HpCDDs) > hexachlorodibenzo-p-dioxins (HxCDDs) > pentachlorodibenzo-p-dioxins (PeCDDs) > tetrachlorodibenzo-p-dioxins (TeCDDs). The dioxin sources may be distinguished according to a ratio analysis of PCDD to PCDF and 1,2,3,7,8,9-HxCDD to 1,2,3,6,7,8-HxCDD.3,5 Table 4 lists the concentration ratios of PCDD to PCDF and 1,2,3,7,8,9-HxCDD to 1,2,3,6,7,8HxCDD determined in the clays in this study. Horii et al.5 have found that the ratio of PCDD:PCDF in the kaolin (1040− 8600) was much higher than the ratios detected in anthropogenic sources (range: 0.7−200). It has also been demonstrated that the PCDD:PCDF ratios in raw and processed American ball clays were 957 and 2672, respectively.3 Generally, the ratio of 1,2,3, 7,8,9-HxCDD:1,2,3,6,7,8-HxCDD was >1 in kaolins, whereas the ratio 1200 °C) in a kiln during the firing process. However, one portion of the dioxins might volatilize and evaporate into the atmosphere before the temperature was elevated to a level high enough for their destruction. Before firing, ceramic ware are dried and preheated at various temperatures (usually
