Environ. Sci. Technol. 2007, 41, 957-962
Inhibition of Polybrominated Dibenzo-p-dioxin and Dibenzofuran Formation from the Pyrolysis of Printed Circuit Boards YI-CHIEH LAI, WEN-JHY LEE,* AND HSING-WANG LI Department of Environmental Engineering and Sustainable Environmental Research Center, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China LIN-CHI WANG AND GUO-PING CHANG-CHIEN Department of Chemical and Materials Engineering, Cheng Shiu University, Kaohsiung 833, Taiwan, Republic of China
Waste printed circuit boards containing brominated flame retardants were pyrolyzed in a high-temperature melting system to observe the formation behaviors of polybrominated dibenzo-p-dioxins (PBDDs) and dibenzofurans (PBDFs). In this study, the results showed that the formation of PBDD/ Fs during pyrolysis can be destroyed under controlled primary combustion conditions. There were two significant factors that influenced the extent of PBDD/F formation. The first factor was temperature. The results showed that, both the total PBDD/F content in the bottom ash and the total PBDD/F emission factor from the flue gas decrease by approximately 50% with an increase of the pyrolysis temperature from 850 to 1200 °C. The second factor was the addition of CaO. The possible mechanism involves the reaction between CaO and HBr to form the solid-phase product CaBr2. Thus, the addition of CaO is effective in adsorbing HBr and results in the inhibition of PBDD/F synthesis by more than 90% and further prevents the acid gases (HCl and HBr) that corrode the equipment. In conclusion, due to the persistence and toxicity of PBDD/Fs, a combined regulation for controlling both PCDD/Fs and PBDD/Fs is of great importance for environmental protection issues.
Introduction The growth of the electronics industry has stimulated the demand for printed circuit boards (P-CBs) and thus impelled the expansion of the P-CB industry. In 2003, the global P-CB output was 222.4 Mm2, and the total value reached $33.3 billion (1). In general, P-CB consists of electronic parts, a glass-fiber resin substrate, and a copper sheet (2). Brominated flame retardants (BFRs) are widely used in P-CBs because of their great flame-retardant properties and low cost (3-5). The commonly used BFRs are polybrominated diphenyl ethers, tribromophenol, decabromobiphenyl, and tetrabromobisphenol A (3, 5). With increasing use of BFR products, the environmental concern about the formation of polybrominated dibenzo-p-dioxins (PBDDs) and dibenzofurans (PBDFs) has become greater than ever before. The major * Corresponding author phone: +886-6-2757575 ext 65831; fax: +886-6-2752790; e-mail:
[email protected]. 10.1021/es061420c CCC: $37.00 Published on Web 12/14/2006
2007 American Chemical Society
formation of PBDD/Fs is as byproducts in the manufacture of BFRs and during thermal treatment of products containing BFRs (3). At present, several technologies have been applied to dispose of waste P-CBs, including chemical methods (such as hydrometallurgical processes), physical methods (such as magnetic or gravity separation), and thermal methods (such as pyrolysis, combustion, or incineration) (2, 7). In recent years, thermal methods have been the most widely applied technology. The high Cl and Br contents in P-CBs not only are the major emission source of PBDD/Fs, but also release acid gas (HCl and HBr) that corrodes equipment under thermal treatment. Due to the similar properties of chlorine and bromine, the formation mechanisms of PBDD/Fs may be comparable with those of PCDD/Fs during thermal treatment; e.g., PBDD/ Fs are formed (a) through de novo synthesis and/or (b) from precursor compounds (such as brominated aromatic compounds) during condensation or elimination reactions (8). The World Health Organization reported that PBDD/Fs are pollutants similar to PCDD/Fs in their persistence and toxicity and that humans and the environment should be protected from these compounds (9, 10). There are many cases of PBDD/Fs being detected in flue gas and fly ashes from municipal solid waste incinerators (3, 11, 12), but fewer studies of the behavior of PBDD/Fs from laboratory thermolysis of BFR products have been reported. For example, only concentrations for TV casting (3-66 µg/g) and TV P-CBs (38-130 µg/g) were reported in Japan (13). Furthermore, the concentration in waste electric and electronic equipment (12.3 ng international toxicity equivalency quantity (I-TEQ)/Nm3) by incineration of halogen-rich fuel was reported (14). In addition, the capability of different air pollution control devices (APCDs) for the removal of PBDD/ Fs is unknown. The distribution of PBDD/Fs in different media (aqueous, particles, and gaseous) needs to be quantified to determine emission factors from three different APCDs (cooling unit, filter, and glass cartridge). Since the use of lime is noted for the removal of gaseous PCDD/Fs (15), its use in reducing PBDD/Fs may yield similar results. The installation of another lime adsorber may be too costly; thus, the direct addition of lime on feeds has never been investigated. Consequently, the present study was undertaken to address these concerns. High-temperature melting equipment was used. Three different APCDs were installed to observe emission factors and removal efficiencies. Two pyrolysis temperatures were used to observe its effect on the overall PBDD/F formation. Raw lime (CaO) was added to the feed to observe its effect on the overall PBDD/F formation.
Experimental Section P-CBs. The waste flexible P-CBs which were used in mobile phones were obtained from the Waste Management Plant in Taiwan. The contents of C, H, and N in P-CBs were analyzed with an elemental analyzer (Vario EL), those of S and Cl were analyzed with a sulfur/chlorine analyzer (TOX-100), and that of Br was analyzed with an X-ray fluorescence spectrometer (Spectro XRF-XEPOS). The waste flexible P-CBs were first crushed into small pieces (ca. 1 × 1 × 1 mm) by a crusher (Retsch DM200). The chemical compositions of P-CBs were determined by microwave-assisted acid digestion based on the National Environment Analysis Method of Taiwan. The digestion was carried out with 0.05 g of the sample in aqua regia (20 mL), and then the metal concentrations in the digesting solution were measured by inductively coupled VOL. 41, NO. 3, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Mean Elemental and Main Metal Contents in Printed Circuit Boards (n ) 3) element content (wt %)
content RSDa (%) extraction solution RSD (%) a
C
N
S
Cl
Br
Al
Ni
Cu
Au
3.25 18.8
47.5 25.8
3.69 23.4
0.21 17.5
1.01 43.2
4.14 69.8
0.87 31.5 0.74 23.5
0.86 50.3 0.81 18.8
41.9 13.7 31.4 19.1
0.028 3.20 0.023 15.1
RSD ) relative standard deviation.
FIGURE 1. High-temperature melting system: (1) bottom ash, (2) cooling unit, (3) filter, (4) glass PUF cartridge. plasma atomic emission spectrometry (ICP-AES) (Jobin-Yvon JY-38 Plus ICP-AES instrument). Elemental analyses of the waste P-CBs, including Ag, Al, As, Au, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, Pb, Sb, Se, Sn, Sr, Ti, V, and Zn, were determined. Except for the main metals (Al, Cu, Ni, and Au), the concentration of the other 18 elements was lower than 0.01%. Therefore, we only summarized the main metal (Al, Cu, Ni, and Au) contents in Table 1. The mean contents of moisture, ash, and combustibles in P-CBs were 0.8%, 43%, and 56%, respectively. The results of elemental analyses (C, H, N, S, Cl, and Br) and main metal (Al, Cu, Ni, and Au) contents are summarized in Table 1. Note that the Cu content was 42%. Laboratory Melting System. The melting system consists of a high-temperature furnace, a secondary combustion chamber, and one set of APCDs including a cooling unit (collects the condensed water of the volatile gas), a filter (collects particles), and three-stage glass PUF cartridge adsorption (collects the gaseous phase compound) as illustrated in Figure 1. The specifications of the melting system are as follows. The high-temperature furnace has a width of 210 mm, a length of 280 mm, and a height of 220 mm, with a maximum temperature of 1600 °C, and the heat loading is 6.6 kW. The furnace has an inner graphite crucible, and on top of this crucible is a cone-shaped cap hood for collecting volatile gases. The volatile gases collected by the cap hood will be delivered with an Al2O3 tube to the secondary combustion chamber for further combustion or introduced to the APCDs. The secondary combustion chamber has an inside diameter of 50 mm and a length of 1000 mm, with a maximum temperature of 1300 °C, and the heat loading is 6.6 kW. The flue gas cooling unit with indirect water cooling has an inside diameter of 25 mm and a length of 1700 mm. The diameter of the filter is 51 mm, and each stage of the three-stage PUF cartridge adsorption has a length of 130 mm. To ensure that the gaseous PBDD/Fs are insignificant, three separate tests were performed by measuring PBDD/Fs in the third stage of the PUF cartridge, and less than 2% PBDD/F mass was found at the third stage of the PUF cartridge. The waste flexible P-CBs were first cut into small pieces (ca. 5 × 5 × 0.5 mm), and metals on the surface were stripped 958
main metal content (%)
H
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off by aqua regia extraction. The waste P-CB was pretreated to prevent the metals from corroding the equipment. The concentrations of Al, Cu, Ni, and Au in the extraction solution were analyzed with the same ICP-AES instrument. When the metals were stripped off the surface of the waste P-CB by the aqua regia solution, the concentration of metals was reduced by 75-95% and the weight of the P-CB reduced by approximately 25% (Table 1). The experimental sample placed inside a graphite crucible was put in the furnace; the furnace temperature was than increased to 850 or 1200 °C at 6 °C min-1 (held for 30 min), with the secondary combustion chamber at 1200 °C. Two samples were used: (i) 30 g of flexible P-CBs and (ii) 30 g of flexible P-CBs and 30 g of CaO at 850 °C (denoted as P1 or A1) and 1200 °C (denoted as P2 or A2), respectively. The pump rate for withdrawing gaseous samples was 10 L min-1. The batch experiment was repeated twice. After the pyrolysis process, the bottom ash, fly ash, and cartridge were collected and analyzed to determine the concentrations of PBDD/Fs on the basis of the National Environment Analysis Method (NIEA M801.11B) of Taiwan, similar to USEPAB modified method 23A. PBDD/F Analysis. Analyses of PBDD/F samples were performed in a certified laboratory in Taiwan to analyze PCDD/Fs and PBDD/Fs. Each sample was spiked with a known standard and extracted for 24 h. Then the extract was concentrated and treated with sulfuric acid; this was followed by a series of cleanup and fraction procedures. The standard solution was added to the sample before PBDD/F analysis to ensure the recovery during analysis. Because of a lack of other standards, seven individual PBDD/Fs (including four individual PBDDs and three individual PBDFs) were analyzed by using high-resolution gas chromatography/mass spectrometry (HRGC/MS). The HRGC instrument (HewlettPackard 6970 series gas, California) was equipped with an RTX-5MS column (L ) 30 m, i.d. ) 0.25 mm, film thickness 0.25 µm) and splitless injection (J&W Scientific, California). The oven temperature was programmed according to the following: initial temperature at 150 °C (held for 1 min), increased to 220 °C at 40 °C min-1, then increased to 240 °C at 2 °C min-1, and then increased to 310 °C at 10 °C min-1 (held for 1 min). Helium was used as the carrier gas. The HRMS instrument (Micromass Autospec Ultima, Manchester, U.K.) was equipped with a positive electron impact (EI+) source. The analyzer mode was selected ion monitoring with a resolving power of 10000. The electron energy and the source temperature were set at 35 eV and 250 °C, respectively. The method detection limits of the seven individual PBDD/ Fs for bottom ash samples, cooling unit samples, filter samples, and glass PUF cartridge samples were found between 0.318 and 4.132 ng/kg, 0.001 and 0.008 ng, 0.001 and 0.021 ng/Nm3, and 0.004-0.071 ng/Nm3, respectively. The recovery for the seven individual PBDD/Fs compounds ranged from 50% to 107%.
Results and Discussion Effect of Temperature on the PBDD/F Formation. This experiment investigated the difference in behavior of PBDD/
TABLE 2. Mean PBDD/F Contents in Bottom Ashes (n ) 2) content (ng/g) compound
P1 (850 °C)
P2 (1200 °C)
2,3,7,8-TeBDD 1,2,3,7,8-PeBDD 1,2,3,4/6,7,8-HxBDD 1,2,3,7,8,9-HxBDD 2,3,7,8-TeBDF 1,2,3,7,8-PeBDF 2,3,4,7,8-PeBDF PBDDs PBDFs total PBDD/Fs total I-TEQ
0.013 0.147 0.336 0.112 0.180 0.417 0.638 0.609 1.23 1.84 0.490
0.002 0.065 0.311 0.102 0.024 0.137 0.293 0.480 0.454 0.935 0.231
properties of chlorine and bromine, the above results were similar to those obtained by Kim et al. (16). They reported that the higher chlorinated congeners were predominant in the bottom ashes. Due to the incomplete research on the toxic effects of PBDD/Fs, there is still no complete toxicity equivalency factor (TEF) value. For the present survey of several biological and toxicological parameters for animals, 2,3,7,8-TeBDD and 2,3,7,8-TeBDF were equipotent to 2,3,7,8-TeCDD and 2,3,7,8TeCDF. To assess the toxicologically relevant information on PBDD/Fs, WHO (17) recommends the use of the same TEF values for PBDD/Fs as described for the chlorinated analogues. According to this criterion, we calculate the I-TEQ value for PBDD/Fs by using the concentrations of seven 2,3,7,8- brominated substitutes and the TEF of their chlorinated analogues. The mean total PBDD/F (summation of seven individual PBDD/Fs) contents and total TEQ concentrations (summation of seven individual TEQs) in the bottom ashes of P1 (850 °C) and P2 (1200 °C) were 1.84 and 0.935 ng/g and 0.490 and 0.231 ng I-TEQ/g, respectively (Table 3). The RSDs of seven individual PBDD/Fs in the bottom ashes of P1 ranged from 30.5% (2,3,7,8-TeBDF) to 47.3% (1,2,3,4/ 6,7,8-HxBDD), and those of P2 ranged from 41.7% (1,2,3,7,8PeBDD) to 69.3% (1,2,3,4/6,7,8-HxBDD). Nevertheless, there were still no restrictions for the emission of 2,3,7,8-brominated substituted dioxins and furans. Furthermore, the above results also show that about 83% of the total PBDD/F is adsorbed on the surface of bottom ashes and 17% is found in the flue gas. The reason is that PBDD/Fs have higher molecular weights and lower vapor pressures (18), resulting in the high PBDD/F contents in the bottom ashes. On the other hand, the high copper content (7.8%) in the experimental sample may act as the catalyst and enhance the formation of PBDD/Fs through a surface-catalyzed mechanism. The above results were similar to those obtained by Sakai et al. (3). They reported that the presence of metals (such as Fe and Cu) tends to promote formation of PBDD/ Fs. Last, when compared with the mean total PBDD/F content of P1 (850 °C), a higher temperature of P2 (1200 °C) did reduce the total PBDD/F contents in the bottom ashes by approximately 50% (Table 2). It was generally observed that certain factors influence the extent of PBDD/F formation, including the temperature and duration of the process. The low-temperature combustion results in relatively high emissions of PBDD/Fs. From the results of this study and other research, brominated aromatics can be supposed to be destroyed faster under thermal treatment, resulting in less PBDD/Fs (8, 19, 20).
FIGURE 2. Congener profiles of seven 2,3,7,8-brominated substituted PBDD/Fs detected from the bottom ashes of P1 and P2. Fs released at different temperatures (850 and 1200 °C) in the primary furnace. Table 2 lists the PBDD/F contents in the bottom ashes. The results revealed that the PBDD/F concentrations increased with increasing number of brominated substitutes at temperatures of both 850 and 1200 °C. Figure 2 shows the congener profiles of seven 2,3,7,8brominated substituted PBDD/Fs detected from the bottom ashes of P1 and P2. The top two congeners for both samples P1 and P2 were 2,3,4,7,8-PeBDF and 1,2,3,4/6,7,8-HxBDD. This shows that the higher bromine-substituted PBDD/Fs with lower vapor pressures would be less mobile and thus were predominant in the bottom ashes. Due to the similar
Emission Factors. To assess the PBDD/F distribution in APCDs, this study shows the emission factors of seven individual PBDD/Fs per gram of waste P-CB being pyrolyzed, which were collected by the cooling unit, filter, and glass PUF cartridge (Table 3). The result shows that the emission factors of total PBDD/Fs collected by the cooling unit, filter, and glass PUF cartridge were 214, 12.0, and 143 pg/g of waste in P1 and 73.5, 13.3, and 99.5 pg/g of waste in P2, respectively. The RSDs of seven individual PBDD/Fs in P1 and P2 ranged from 50.1% to 141%. This shows that the cooling unit collected the condensed water of volatile gas and thus has the highest emission factor of total PBDD/Fs. On the other hand, particles in the flue gas were collected on quartz filters (Pallflex Tissuquartz 2500 QAT-UP). The filters allow the collection of particles in coarse (Dp > 0.5 µm) modes, and those in fine (Dp < 0.5 µm) modes were captured and adsorbed by the glass PUF cartridge. Thus, the resulting emission factor of total PBDD/Fs collected by the glass PUF cartridge was higher by 1 order of magnitude than that of the PBDD/Fs collected by the filter. VOL. 41, NO. 3, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Mean Emission Factors (pg/g of waste) of PBDD/Fs Collected by the Cooling Unit, Filter, and Glass PUF Cartridge (n ) 2) cooling unit
a
filter
glass PUF cartridge
compound
P1 (850 °C)
P2 (1200 °C)
P1 (850 °C)
P2 (1200 °C)
P1 (850 °C)
P2 (1200 °C)
2,3,7,8-TeBDD 1,2,3,7,8-PeBDD 1,2,3,4/6,7,8-HxBDD 1,2,3,7,8,9-HxBDD 2,3,7,8-TeBDF 1,2,3,7,8-PeBDF 2,3,4,7,8-PeBDF PBDDs PBDFs total PBDD/Fs
15.3 57.6 66.2 26.7 15.1 9.51 23.8 166 48.0 214
3.62 21.3 23.3 7.47 4.39 4.39 8.98 55.7 17.8 73.5
BDa 1.73 7.32 2.39 BD BD 0.557 11.4 0.557 12.0
0.056 2.06 7.10 2.61 0.053 0.387 1.04 11.8 1.50 13.3
4.03 31.1 54.1 20.7 9.25 7.89 16.1 110 33.0 143
6.89 27.3 34.9 11.2 3.79 4.53 10.9 80.3 19.2 99.5
BD ) below detection limit
TABLE 4. Mean PBDD/F Contents in Bottom Ashes by Adding CaO during Pyrolysis (n ) 2) content (ng/g) compound
A1 (850 °C)
A2 (1200 °C)
2,3,7,8-TeBDD 1,2,3,7,8-PeBDD 1,2,3,4/6,7,8-HxBDD 1,2,3,7,8,9-HxBDD 2,3,7,8-TeBDF 1,2,3,7,8-PeBDF 2,3,4,7,8-PeBDF PBDDs PBDFs total PBDD/Fs total I-TEQ
0.001 0.001 0.075 0.009 0.003 0.023 0.060 0.085 0.086 0.171 0.040
0.001 0.001 0.005 0.001 0.004 0.003 0.016 0.007 0.022 0.029 0.010
experiment investigated the effect of adding calcium oxide (CaO) to the waste P-CB on the PBDD/F control. The CaO was added to the waste P-CB in a ratio of one (P-CB:CaO ) 1:1). The congener profiles of seven 2,3,7,8-brominated substituted PBDD/Fs detected from the bottom ashes of A1 and A2 are shown in Figure 3. This figure shows that 2,3,4,7,8PeBDF and 1,2,3,4/6,7,8-HxBDD were predominant in the bottom ashes. Table 4 shows the total PBDD/F (summation of seven individual PBDD/Fs) contents and totalTEQ concentrations (summation of seven individual TEQs) in the bottom ashes of A1 (850 °C) and A2 (1200 °C) were 0.171and 0.029 ng/g and 0.040 and 0.010 ng I-TEQ/g, respectively. The RSDs of seven individual PBDD/Fs in the bottom ashes of A1 and A2 ranged from 14.6% to 57.1%. When compared with those of the samples with no CaO present (Table 2), the total PBDD/Fs were decomposed by 1-2 orders of magnitude. In general, acid gases (HBr) were generated during thermolysis in the presence of bromine (22, 23). A possible neutralization between CaO and HBr could explain these changes. The possible mechanism between CaO and HBr is as follows: FIGURE 3. Congener profiles of seven 2,3,7,8-brominated substituted PBDD/Fs detected from the bottom ashes of A1 and A2.
CaO + 2HBr f CaBr2 + H2O
In addition, the above results indicated that an increase of the pyrolysis temperature of the furnace from 850 to 1200 °C not only decreased the total PBDD/F content in the bottom ash, but also reduced the total PBDD/F emission factor from the three different APCDs. It has been shown that the PBDD/ Fs formed during pyrolysis can be destroyed under controlled primary combustion conditions. PBDD/F Inhibition by Adding Calcium Oxide. Lime is noted as an alkaline sorbent for dechlorination for the removal of gaseous PCDD/Fs (15, 21). For this reason, this
Equation 1 shows that the reaction between HBr and CaO formed the solid-phase product CaBr2, which was less volatile and was collected in the bottom ashes. Thus, the addition of CaO is effective to adsorb HBr, resulting in the inhibition of PBDD/F synthesis. The above results are similar to those obtained by Tagashira et al. (21). They reported that Ca(OH)2 was added for dechlorination and was effective in inhibiting dioxin synthesis. Table 5 shows the emission factor of seven individual PBDD/Fs per gram of waste printed circuit board being
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(1)
TABLE 5. Mean Emission Factors (pg/g of waste) of PBDD/Fs Collected by the Cooling Unit, Filter, and Glass PUF Cartridge by Adding CaO during Pyrolysis (n ) 2) cooling unit
a
filter
glass PUF cartridge
compound
A1 (850 °C)
A2 (1200 °C)
A1 (850 °C)
A2 (1200 °C)
A1 (850 °C)
A2 (1200 °C)
2,3,7,8-TeBDD 1,2,3,7,8-PeBDD 1,2,3,4/6,7,8-HxBDD 1,2,3,7,8,9-HxBDD 2,3,7,8-TeBDF 1,2,3,7,8-PeBDF 2,3,4,7,8-PeBDF PBDDs PBDFs total PBDD/Fs
0.023 0.038 0.143 BD 0.754 BD BD 0.204 0.754 0.958
0.010 0.043 0.108 0.041 0.010 0.198 0.189 0.203 0.397 0.600
BDa BD BD BD 0.038 BD BD 0.038 0.000 0.038
BD BD BD BD 0.017 BD BD 0.000 0.017 0.017
0.043 0.146 BD BD 0.153 0.083 0.120 0.189 0.353 0.543
BD BD BD BD 0.037 BD BD 0.000 0.037 0.037
BD ) below detection limit
TABLE 6. Removal Efficiencies (%) of PBDD/Fs in the Cooling Unit and Filter (n ) 2) cooling unit
2,3,7,8-TeBDD 1,2,3,7,8-PeBDD 1,2,3,4/6,7,8-HxBDD 1,2,3,7,8,9-HxBDD 2,3,7,8-TeBDF 1,2,3,7,8-PeBDF 2,3,4,7,8-PeBDF PBDDs PBDFs total PBDD/Fs
filter
P1 (850 °C)
P2 (1200 °C)
A1 (850 °C)
A2 (1200 °C)
P1 (850 °C)
P2 (1200 °C)
A1 (850 °C)
A2 (1200 °C)
79 64 52 54 62 55 59 58 59 58
34 42 36 35 53 47 43 38 46 40
35 21 ∼100 ∼0.0 80 ∼0.0 ∼0.0 47 68 62
∼100 ∼100 ∼100 ∼0.0 16 ∼100 ∼100 79 ∼100 92
∼0.0 5.3 12 10 ∼0.0 ∼0.0 3.3 9.4 1.7 7.7
0.8 7.0 17 19 1.4 7.9 8.7 13 7.1 12
∼0.0 ∼0.0 ∼0.0 ∼0.0 20 ∼0.0 ∼0.0 17 ∼0.0 6.5
∼0.0 ∼0.0 ∼0.0 ∼0.0 32 ∼0.0 ∼0.0 32 ∼0.0 32
pyrolyzed by adding CaO and collected by the cooling unit, filter, and PUF/resin cartridge. The result shows that the emission factors of total PBDD/Fs collected by the cooling unit, filter, and glass PUF cartridge were 0.958, 0.038, and 0.543 pg/g of waste in A1 and 0.600, 0.017, and 0.037 pg/g of waste in A2, respectively. The RSDs of seven individual PBDD/Fs in A1 and A2 ranged from 102% to 141%. This shows that the cooling unit has the highest emission factor of total PBDD/Fs. These results are similar to those shown in Table 3. Last, when compared with those of the samples with no CaO present (Table 3), the emission factors of total PBDD/Fs collected by the cooling unit, filter, and glass PUF cartridge of A1 and A2 were reduced by 2-3 orders of magnitude. The above results indicate that CaO is effective in inhibiting PBDD/F synthesis, not only by decreasing the totalPBDD/F content in the bottom ash, but also by reducing the total PBDD/F emission factor from the flue gas. Removal Efficiency of PBDD/Fs by APCDs. The removal efficiency of PBDD/Fs by APCDs is calculated as follows: removal efficiency (%) in the cooling unit ) A/(A + B + C) × 100%, and removal efficiency in the filter ) B/(B + C) × 100%, where A is the emission factor of PBDD/Fs collected by the cooling unit, B is the emission factor of those collected by the filter, and C is the emission factor of those collected by the PUF/resin cartridge. The removal efficiencies of total PBDD/Fs by the cooling unit were 58% in P1, 40% in P2, 62% in A1, and 92% in A2; however, those by the filter were 7.7% in P1, 12% in P2, 6.5% in A1, and 32% in A2 (Table 6). The above results show the removal efficiencies of total PBDD/ Fs by the cooling unit were higher than those by the filter. They also suggest that only using APCDs including a cooling unit, a filter, and a three-stage glass PUF cartridge adsorption is not sufficient for PBDD/F control, and additional APCDs, such as activated carbon injection in front of the filter, are
needed. Due to the persistence and toxicity of PBDD/Fs, a combined regulation for controlling both PCDD/Fs and PBDD/Fs is of great importance for environmental protection.
Acknowledgments We are sincerely grateful to Professor Oliver J. Hao for his insightful discussion. This research was supported in part by the National Science Council of the Taiwan Government via Grant No. NSC-94-2218-E-006-24.
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Received for review June 14, 2006. Revised manuscript received November 2, 2006. Accepted November 2, 2006. ES061420C
